doc/input: Add documention to MPR121 Capacitive Keypad

This commit adds the Documentation to MPR121 Keypad and the board
profile documentation to STM32F4Discovery board.

Signed-off-by: Alan C. Assis <acassis@gmail.com>

Documentation/components/drivers/character/input/index.rst

@@ -7,6 +7,7 @@ Input Devices
 
   keypad-keyboard.rst
   keypad.rst
+  mpr121.rst
   sbutton.rst
 
 See ``include/nuttx/input/*.h`` for registration information.

Documentation/components/drivers/character/input/mpr121.rst

@@ -0,0 +1,72 @@
+========================
+MPR121 Capacitive Keypad
+========================
+
+**What is the MPR121 Keypad**
+The  MPR Capacitive Keypad is a small keyboard with 12 keys, similar
+to a telephone keypad, however the keys are numered from 0 to 11.
+
+This is the picture of the MPR121 Capacitive Keypad:
+
+.. image:: images/mpr121.png
+  :alt: MPR121 Capacitive Keypad
+  :align: center
+  :width: 200px
+
+**Purpose**. The MPR121 driver provides a generic keypad
+implementation for this capacitive keypad. The MPR121 uses the I2C
+bus to setup and read the touched/released key and uses an IRQ pin
+(open drain) to indicate when a key is pressed/released or when some
+unnexpected issue happens (currently not used).
+
+The driver also enables debounce, and emits keyboard events through
+the common keyboard upper-half. This makes the device available as a
+character driver (e.g., ``/dev/keypad0``) using the standard keyboard
+interfaces.
+
+**Driver Overview**. The MPR121 lower-half scans the matrix and calls
+``keyboard_event()`` when it detects a press or release. The keyboard
+upper-half registers the character device at the requested ``devpath``
+and stores events in a circular buffer. Applications read
+``struct keyboard_event_s`` from the device or use the optional
+kbd-codec layer.
+
+**Board Support**. To support the MPR121, a board must provide:
+
+#. **IRQ Definition**
+
+   - Define the GPIO Input pin with pull-up enabled (since the
+     IRQ pin is open-drain) to be used to detect the interrupts.
+
+#. **Registration Hook**
+
+   - Define a ``struct xxx_mpr121config_s`` that will wrap the
+     ``struct mpr121_config_s config`` as first member and the irq
+     argument and ISR handler (xcpt_t isr). The ``config`` needs to
+     have its ``.irq_attach`` and ``keymap`` initialized with the
+     board IRQ ISR function and the key mapping array.
+   - Implement ``board_mpr121_initialize(int devno, int busno)`` to
+     call ``mpr121_register(&config, devpath)``. It needs to initialize
+     the I2C Master port/channel that will be used to communicate with
+     the MPR121 and an instance of that ``xxx_mpr121config_s`` structure.
+   - Invoke the board hook during bring-up (for example,
+     ``board_mpr121_initialize(0, 1)`` for ``/dev/keymap0`` and ``i2c1``).
+
+**Reference Implementation (STM32F4Discovery)**. The current reference
+is in ``boards/arm/stm32/common/src/stm32_mpr121.c``:
+
+- Keymap: 4x3 keypad layout
+- Registration: ``board_mpr121_initialize()`` calls
+  ``mpr121_register()``
+
+**Data Path Summary**.
+
+- Board calls ``board_mpr121_initialize(0, 1)``
+- ``mpr121_register()`` initializes and configure the MPR121 chip
+  and calls ``keyboard_register(&lower, devpath, buflen)``
+- The upper-half registers the device node at ``devpath``
+- Every time an interruption is generated on IRQ pin it will call the
+  ``mpr121_worker()`` and it is calls ``keyboard_event()`` on
+  press/release of keys.
+- Applications read events from the device node (``/dev/keypad0``).
+