Documentation: RISC-V: Mention the UEFI Standards

Documentation: RISC-V: Mention the UEFI Standards

The current patch acceptance policy requires that specifications are
approved by the RISC-V foundation, but we rely on external
specifications as well.  This explicitly calls out the UEFI
specifications that we're starting to depend on.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20221207020815.16214-4-palmer@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>