doc: sophgo: milkv_duo: document Milk-V Duo board

Add document for Milk-V Duo board which based on Sophgo's CV1800B SoC.

Signed-off-by: Kongyang Liu <seashell11234455@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>

doc/board/index.rst

@@ -46,6 +46,7 @@ Board-specific doc
    sifive/index
    sipeed/index
    socionext/index
+   sophgo/index
    st/index
    starfive/index
    ste/index

doc/board/sophgo/index.rst

@@ -0,0 +1,8 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Sophgo
+======
+.. toctree::
+   :maxdepth: 1
+
+   milkv_duo

doc/board/sophgo/milkv_duo.rst

@@ -0,0 +1,64 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Milk-V Duo
+==========
+
+CV1800B RISC-V SoC
+------------------
+The CV1800B is a high-performance, low-power 1+1 64-bit RISC-V SoC from Sophgo.
+
+Mainline support
+----------------
+The support for following drivers are already enabled:
+1. ns16550 UART 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 milkv_duo_defconfig
+   make
+
+This will generate u-boot-dtb.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.
+
+Alternatively, to run u-boot-dtb.bin on top of FSBL, follow these steps:
+
+1. Use the vendor-provided tool to create a unified fip.bin file containing
+   FSBL, OpenSBI, and U-Boot.
+
+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.
+
+
+Sample boot log from Milk-V Duo board
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+.. code-block:: none
+
+   U-Boot 2024.01-rc5-00010-g51965baa36 (Dec 28 2023 - 13:15:53 +0800)milkv_duo
+
+   DRAM:  63.3 MiB
+   Core:  10 devices, 8 uclasses, devicetree: separate
+   Loading Environment from nowhere... OK
+   In:    serial@4140000
+   Out:   serial@4140000
+   Err:   serial@4140000
+   Net:   No ethernet found.
+   milkv_duo# cpu detail
+     0: cpu@0      rv64imafdc
+      ID = 0, freq = 0 Hz: L1 cache, MMU
+   milkv_duo#