Ox64 BL808 crashes with a Page Fault when we run `getprime` then `hello`. This is caused by the T-Head C906 MMU incorrectly accessing the MMU Page Tables of the Previous Process (`getprime`) while starting the New Process (`hello`).
To fix the problem, this PR flushes the MMU Cache whenever we point the MMU SATP Register to the New Page Tables. We execute 2 RISC-V Instructions that are specific to T-Head C906:
- DCACHE.IALL: Invalidate all Page Table Entries in the D-Cache
- SYNC.S: Ensure that all Cache Operations are completed
This is derived from the T-Head Errata for Linux Kernel. More details here: https://lupyuen.github.io/articles/mmu#appendix-flush-the-mmu-cache-for-t-head-c906
Modified Files:
- `arch/risc-v/src/common/riscv_mmu.h`: If needed, `mmu_write_satp()` calls `mmu_flush_cache()` (weak function) to flush the MMU Cache. (Like for T-Head C906)
- `arch/risc-v/src/bl808/bl808_mm_init.c`: Flush the MMU Cache for T-Head C906. Extend `mmuflags` from 32-bit to 64-bit to be consistent with `mmu_ln_setentry()`.
- `boards/risc-v/bl808/ox64/configs/nsh/defconfig`: Enable `ostest` in the Build Config. Update `CONFIG_BOARD_LOOPSPERMSEC` according to `calib_udelay`.
This PR adds support for PINE64 Ox64 64-bit RISC-V SBC, based on Bouffalo Lab BL808 SoC (T-Head C906 Core). Most of the code is derived from NuttX for Star64 JH7110. The source files are explained in the articles here: https://github.com/lupyuen/nuttx-ox64
### Modified Files
`boards/Kconfig`: Added Ox64 board
### New Files in boards/risc-v/bl808/ox64
`src/bl808_appinit.c`: Startup Code
`include/board.h`: Ox64 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`: Ox64 Makefile
`src/Makefile`: Ox64 Makefile
`Kconfig`: Ox64 Config
`configs/nsh/defconfig`: Build Config for `ox64:nsh`
### Updated Documentation
`platforms/risc-v/bl808/index.rst`: New page for Bouffalo Lab BL808 SoC
`platforms/risc-v/bl808/boards/ox64/index.rst`: Building and booting NuttX for Ox64
`platforms/risc-v/jh7110/boards/star64/index.rst`: Fix typo
