riscv: add documentation for stages and payloads

Change-Id: Iff522e309e9cf9a31c1c79c24047d83d7fd0b00a
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: https://review.coreboot.org/28619
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
1 file changed