doc: falcon: riscv: Falcon Mode boot on RISC-V

Add documentation to introduce the Falcon Mode on RISC-V.
In this mode, the boot sequence is SPL -> OpenSBI -> Linux kernel.

Signed-off-by: Randolph <randolph@andestech.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
1 file changed