doc: add LicheeRV Nano and SG2002 SoC

Provide a page describing the usage of U-Boot on the LicheeRV Nano and a
description of the board.

Signed-off-by: Thomas Bonnefille <thomas.bonnefille@bootlin.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
(cherry picked from commit 62181bbf71)

doc/board/sophgo/index.rst

@@ -6,3 +6,4 @@ Sophgo
    :maxdepth: 1
 
    milkv_duo
+   licheerv_nano

doc/board/sophgo/licheerv_nano.rst

@@ -0,0 +1,72 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+LicheeRV Nano
+=============
+
+SG2002 RISC-V SoC
+-----------------
+The SG2002 is a high-performance, low-power 64-bit RISC-V/ARM SoC from Sophgo.
+
+Mainline support
+----------------
+The support for following drivers are already enabled:
+1. ns16550 UART Driver.
+2. Synopsys Designware MSHC Driver
+
+Building
+~~~~~~~~
+1. Add the RISC-V toolchain to your PATH.
+2. Setup ARCH & cross compilation environment variable:
+
+.. code-block:: console
+
+   export CROSS_COMPILE=<riscv64 toolchain prefix>
+   cd <U-Boot-dir>
+   make sipeed_licheerv_nano_defconfig
+   make
+
+This will generate u-boot.bin
+
+Booting
+~~~~~~~
+Currently, we rely on vendor FSBL (First Stage Boot Loader) to initialize the
+clock and load the u-boot image, then bootup from it.
+
+To run u-boot.bin on top of FSBL, follow these steps:
+
+1. Use mainline OpenSBI with a newer version than 1.5 to generate fw_dynamic.
+
+2. Generate a compatible u-boot.bin using U-Boot with the LicheeRV Nano default
+   configuration.
+
+3. Use the vendor-provided tool [1] to create a unified fip.bin file containing
+   FSBL, OpenSBI, and U-Boot.
+   Note that you will have to use the file cv181x.bin as the FSBL.
+
+2. Place the generated fip.bin file into the FAT partition of the SD card.
+
+3. Insert the SD card into the board and power it on.
+
+The board will automatically execute the FSBL from the fip.bin file.
+Subsequently, it will transition to OpenSBI, and finally, OpenSBI will invoke
+U-Boot.
+
+[1]: https://github.com/sophgo/fiptool
+
+
+Sample boot log from LicheeRV Nano board
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+.. code-block:: none
+
+   U-Boot 2024.10 (Oct 24 2024 - 15:00:20 +0200)licheerv_nano
+
+   DRAM:  256 MiB
+   Core:  19 devices, 11 uclasses, devicetree: separate
+   MMC:   mmc@4310000: 0
+   Loading Environment from nowhere... OK
+   In:    serial@4140000
+   Out:   serial@4140000
+   Err:   serial@4140000
+   Net:   No ethernet found.
+   Hit any key to stop autoboot:  0
+   licheerv_nano#