doc: board: voyager: Add documentation for Voyager

Add documentation for Voyager board.

Signed-off-by: Randolph Sheng-Kai Lin <randolph@andestech.com>
Signed-off-by: Leo Yu-Chi Liang <ycliang@andestech.com>

doc/board/andestech/index.rst

@@ -8,3 +8,4 @@ Andes Tech
 
    adp-ag101p
    ae350
+   voyager

doc/board/andestech/voyager.rst

@@ -0,0 +1,81 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Voyager
+=======
+
+Qilai RISC-V SoC
+----------------
+The QiLai SoC chip is Andes' first RISC-V SoC. It includes high performance
+quad-core Andes AX45MP cluster and one NX27V vector processor.
+
+The Voyager development platform is based on Qilai and capable of running Linux.
+
+Mainline support
+----------------
+
+The support for following drivers are already enabled:
+
+1. UART driver
+2. MMC driver
+3. SPI driver
+
+Building
+~~~~~~~~
+
+How to build U-Boot SPL
+~~~~~~~~~~~~~~~~~~~~~~~
+Before building U-Boot SPL, OpenSBI must be build first.
+OpenSBI can be cloned and build for Voyager as below:
+
+1. Get the RISC-V toolchain.
+2. Setup cross compilation environment variable.
+
+.. code-block:: none
+
+    git clone https://github.com/riscv-software-src/opensbi.git
+    cd opensbi
+    make PLATFORM=generic
+
+Copy OpenSBI FW_DYNAMIC image (build/platform/generic/firmware/fw_dynamic.bin)
+into U-Boot root directory, then
+
+.. code-block:: console
+
+   export CROSS_COMPILE=riscv64-linux-gnu-
+   cd <U-Boot-dir>
+   cp fw_dynamic.bin .
+   make voyager_spl_defconfig
+   make
+
+Booting
+~~~~~~~
+
+Currently, we rely on vendor ROM code to initialize the DDR
+and load the u-boot image, then boot from it.
+
+Sample boot log from Voyager board
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. code-block:: none
+
+    U-Boot SPL 2025.10-rc1-00130-ga28bcbba4778-dirty (Aug 06 2025 - 17:46:10 +0800)
+    Trying to boot from RAM
+
+    U-Boot 2025.10-rc1-00130-ga28bcbba4778-dirty (Aug 06 2025 - 17:46:10 +0800)
+
+    CPU:   riscv
+    Model: andestech,ax45
+    DRAM:  16 GiB
+    Core:  25 devices, 14 uclasses, devicetree: board
+    MMC:   mmc@30c00000: 0
+    Loading Environment from SPIFlash... SF: Detected mx25u1635e with page size 256 Bytes, erase size 4 KiB, total 2 MiB
+    *** Warning - bad CRC, using default environment
+
+    In:    serial@30300000
+    Out:   serial@30300000
+    Err:   serial@30300000
+    Net:   No ethernet found.
+    Hit any key to stop autoboot: 0
+    No ethernet found.
+    No ethernet found.
+    RISC-V #