This PR adds support for the SOPHGO SG2000 SoC, based on T-Head C906 64-bit RISC-V Core. This will be used by the upcoming port of NuttX for Milk-V Duo S SBC.
Most of the code was derived from NuttX for Ox64 BL808. The source files are explained in the articles here: https://github.com/lupyuen/nuttx-sg2000
Modified Files in arch/risc-v:
`Kconfig`: Added ARCH_CHIP_SG2000 for SG2000 SoC
New Files in arch/risc-v:
`include/sg2000/chip.h`: SG2000 Definitions
`include/sg2000/irq.h`: External Interrupts
`src/sg2000/chip.h`: Interrupt Stack Macro
`src/sg2000/sg2000_allocateheap.c`: Kernel Heap
`src/sg2000/sg2000_head.S`: Linux Header and Boot Code
`src/sg2000/sg2000_irq.c`: Configure Interrupts
`src/sg2000/sg2000_irq_dispatch.c`: Dispatch Interrupts
`src/sg2000/sg2000_memorymap.h`: Memory Map
`src/sg2000/sg2000_mm_init.c`, `sg2000_mm_init.h`: Memory Mgmt
`src/sg2000/sg2000_pgalloc.c`: Page Allocator
`src/sg2000/sg2000_start.c`: Startup Code
`src/sg2000/sg2000_timerisr.c`: Timer Interrupt
`src/sg2000/hardware/sg2000_memorymap.h`: PLIC and UART Base Address
`src/sg2000/hardware/sg2000_plic.h`: PLIC Register Addresses
`src/sg2000/Kconfig`: SG2000 Config
`src/sg2000/Make.defs`: Makefile
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