This PR adds support for Milk-V Duo S 64-bit RISC-V SBC, based on SOPHGO SG2000 SoC (T-Head C906 Core). Most of the code is derived from NuttX for Ox64 BL808. The source files are explained in the articles here: https://github.com/lupyuen/nuttx-sg2000
Modified Files:
`boards/Kconfig`: Added Milk-V Duo S board
`arch/risc-v/src/sg2000/sg2000_timerisr.c`: Fixed MTIMER_FREQ for sleep() to work correctly
New Files in boards/risc-v/sg2000/milkv_duos:
`src/sg2000_appinit.c`: Startup Code
`include/board.h`: Milk-V Duo S Definitions
`include/board_memorymap.h`: Memory Map
`src/etc/init.d/rc.sysinit`, `rcS`: Startup Script
`src/.gitignore`: Ignore the tmp filesystem
`scripts/ld.script`: Linker Script
`scripts/Make.defs`: Milk-V Duo S Makefile
`src/Makefile`: Milk-V Duo S Makefile
`Kconfig`: Milk-V Duo S Config
`configs/nsh/defconfig`: Build Config for `milkv_duos:nsh`
Updated Documentation:
`platforms/risc-v/sg2000/index.rst`: New page for SOPHGO SG2000 SoC
`platforms/risc-v/sg2000/boards/milkv_duos/index.rst`: Building and booting NuttX for Milk-V Duo S
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