Documentation: risc-v/mpfs: update documentation

Update the risc-v/mpfs documentation to match the current work.

Signed-off-by: Eero Nurkkala <eero.nurkkala@offcode.fi>

Documentation/platforms/risc-v/mpfs/index.rst

@@ -16,23 +16,38 @@ Like: https://xpack.github.io/riscv-none-embed-gcc or https://github.com/sifive/
 Booting
 =======
 
-The NuttX port for now relies on HSS bootloader to carry on some hardware initializations.
+The NuttX works as a standalone operating system that may initialize all the required clocks and
+peripherals including DDR memory. Alternatively, the vendor's HSS bootloader may be used instead to
+perform all the initialization steps.
 
 
 Building and flashing
 =====================
 
+Configure the NuttX project: ``./tools/configure.sh icicle:nsh``
+Run ``make`` to build the project.
+
+If NuttX is built as a standalone OS with the config option CONFIG_MPFS_BOOTLOADER set, it's important
+to note that the image size is very limited: only (128K - 256) bytes. 256 bytes are reserved for the
+header that must be prepended into the NuttX binary. This binary is called hss-envm-wrapper-bm1-dummySbic.bin
+and may be found from the vendor's HSS implementation. Moreover, after prepending the 256-byte header,
+the LMA section needs to be adjusted, for example::
+
+   riscv64-unknown-elf-objcopy -I binary -O ihex --change-section-lma *+0x20220000 nuttx.bin nuttx.hex
+
+The output binary nuttx.hex may be flashed with the LiberoSoc tool.
+The tool is available from: https://www.microchip.com/en-us/products/fpgas-and-plds/fpga-and-soc-design-tools/fpga/libero-software-later-versions#Documents%20and%20Downloads
+
+There's an alternative way to use NuttX, without the CONFIG_MPFS_BOOTLOADER option set. This expects the
+HSS bootloader sets up the system (memories, caches, initializes clocks etc.).
 First make sure that ``hss-payload-generator`` is installed.
 Available from: https://github.com/polarfire-soc/hart-software-services
 
 This tool is used to convert the ELF/bin to a compatible HSS payload image
 
-Configure the NuttX project: ``./tools/configure.sh icicle:nsh``
-Run ``make`` to build the project.
-
 Create HSS payload bin::
 
-  hss-payload-generator -v -c hss-nuttx.yml payload.bin
+   hss-payload-generator -v -c hss-nuttx.yml payload.bin
 
 
 Debugging with OpenOCD
@@ -59,7 +74,7 @@ GPIO           Yes
 MMUART         Yes      Uart mode only
 SPI            Yes      
 I2C            Yes      
-eMMC SD/SDIO   Yes      eMMC not fully tested
+eMMC SD/SDIO   Yes      No PHY training
 Timers         No
 Watchdog       No       
 RTC            No