docs/rc: document RC/LIRC drivers and remove empty drivers/rmt files

Add documentation for the RC/LIRC character driver subsystem covering
device registration, the LIRC interface, and usage from user space.

Remove placeholder empty files under drivers/rmt that were left over
from the rmtchar era and are no longer referenced.

Signed-off-by: Piyush Patle <piyushpatle228@gmail.com>

Documentation/components/drivers/character/rc.rst

@@ -1,3 +1,139 @@
 ======================
 Remote Control Devices
 ======================
+
+NuttX provides a Remote Control (RC) framework for infrared style input and
+output devices. The userspace interface follows the LIRC model and exposes
+each device as a character node such as ``/dev/lirc0``.
+
+Like many NuttX drivers, the RC subsystem is split into two parts:
+
+#. a generic upper half in ``drivers/rc/lirc_dev.c``, and
+#. a lower half supplied by platform or device specific code.
+
+The upper half handles the character driver registration, per-open buffering,
+``poll()`` support, and the common ``ioctl()`` interface. The lower half is
+responsible for the hardware specific work such as receiving pulse timings,
+transmitting IR data, or reporting decoded scancodes.
+
+Files related to the RC framework are located in:
+
+- ``include/nuttx/lirc.h``
+- ``include/nuttx/rc/lirc_dev.h``
+- ``drivers/rc/lirc_dev.c``
+
+Application Programming Interface
+=================================
+
+Applications should include the following header files:
+
+.. code-block:: c
+
+  #include <nuttx/lirc.h>
+  #include <sys/ioctl.h>
+
+Each RC device is registered as a POSIX character device in ``/dev``.
+Applications open the device with the standard ``open()`` call and then use
+``read()``, ``write()``, ``poll()``, and ``ioctl()`` just like other character
+drivers.
+
+The RC framework supports three kinds of lower-half devices:
+
+- ``LIRC_DRIVER_SCANCODE`` for devices that report decoded scancodes
+- ``LIRC_DRIVER_IR_RAW`` for devices that report raw pulse/space timings
+- ``LIRC_DRIVER_IR_RAW_TX`` for transmit-only raw devices
+
+Raw Pulse/Space Format
+======================
+
+Raw IR timing is transferred in the LIRC ``mode2`` format. Each sample is a
+32-bit ``unsigned int`` value. The upper 8 bits describe the sample type and
+the lower 24 bits carry the payload, usually a duration in microseconds.
+
+The helpers in ``include/nuttx/lirc.h`` are intended to build and inspect
+these values:
+
+- ``LIRC_PULSE(usec)``
+- ``LIRC_SPACE(usec)``
+- ``LIRC_FREQUENCY(value)``
+- ``LIRC_TIMEOUT(usec)``
+- ``LIRC_IS_PULSE(sample)``
+- ``LIRC_IS_SPACE(sample)``
+- ``LIRC_VALUE(sample)``
+
+Typical raw transmit data looks like this:
+
+.. code-block:: c
+
+  unsigned int txbuf[] =
+  {
+    LIRC_PULSE(9000),
+    LIRC_SPACE(4500),
+    LIRC_PULSE(560),
+    LIRC_SPACE(560),
+    LIRC_PULSE(560),
+  };
+
+For raw transmit, ``write()`` expects an odd number of ``unsigned int``
+samples. The upper half forwards the pulse/space sequence to the lower-half
+``tx_ir()`` callback.
+
+IOCTL Commands
+==============
+
+The RC upper half supports the standard LIRC ``ioctl()`` commands defined in
+``include/nuttx/lirc.h``. Commonly used commands include:
+
+- ``LIRC_GET_FEATURES``
+- ``LIRC_GET_SEND_MODE``
+- ``LIRC_GET_REC_MODE``
+- ``LIRC_SET_SEND_MODE``
+- ``LIRC_SET_REC_MODE``
+- ``LIRC_GET_REC_RESOLUTION``
+- ``LIRC_SET_SEND_CARRIER``
+- ``LIRC_SET_SEND_DUTY_CYCLE``
+- ``LIRC_SET_REC_TIMEOUT``
+
+Support for a specific command depends on the lower-half capabilities. In
+practice, applications usually start by querying ``LIRC_GET_FEATURES`` and
+then only use the operations that the device advertises.
+
+Lower-Half Registration
+=======================
+
+Platform code registers an RC lower half by filling ``struct lirc_lowerhalf_s``
+and calling ``lirc_register()``.
+
+.. code-block:: c
+
+  int ret = lirc_register(lower, devno);
+
+The lower half provides a ``struct lirc_ops_s`` callback table. Depending on
+the hardware, it may implement open/close callbacks, raw transmit via
+``tx_ir()``, scancode transmit via ``tx_scancode()``, carrier and duty-cycle
+configuration, or receive timeout handling.
+
+Testing
+-------
+
+The ``irtest`` application in ``nuttx-apps/system/irtest`` can be used to
+exercise RC devices from NSH. A typical validation sequence is:
+
+#. open ``/dev/lircN``
+#. query ``LIRC_GET_FEATURES``
+#. query or set send / receive mode
+#. write raw mode2 samples on transmit-capable devices
+#. read back mode2 samples from receive-capable devices
+
+Espressif RMT LIRC Adapter
+==========================
+
+Espressif targets may provide an arch-local LIRC adapter built on top of the
+RMT peripheral:
+
+- ``arch/xtensa/src/common/espressif/esp_lirc.c``
+- ``arch/risc-v/src/common/espressif/esp_lirc.c``
+
+These adapters expose the RMT peripheral through the RC framework as
+``/dev/lircN`` devices while keeping the hardware specific RMT implementation
+in the Espressif lower-half driver.