efiprot.h: add definitions for RISCV_EFI_BOOT_PROTOCOL

It seems like this file was vendored (sorta) originally, but we have not
kept it in sync with the original source, if it still exists. Follow the
precedent in commit acf82d2659879, and just add the definition to the
bottom of the file, keeping its style conventions.

Defined here: https://github.com/riscv-non-isa/riscv-uefi

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48885