Docs for RISC-V

[Simon Glass]

Hi Rick & Leo,

I see docs for one board at [1] but I'm not sure what to do with unleashed.

It seems to need a 'genimage' tool, going on what I see in CI.

Could you adjust it to use binman to output an image suitable for
writing to an SD card in a single 'dd' step, as Rockchip does?

Also, how about adding a RISC-V section here?

https://u-boot.readthedocs.io/en/latest/arch/index.html

Thanks,
Simon

[1] https://u-boot.readthedocs.io/en/latest/board/sifive/unmatched.html