Documentation: add riscv entry in list of existing profiles

As long as there are only a few maintainer entry profiles, i.e., three
in v5.8, continue to maintain a complete a list of entries in the
maintainer handbook.

Complete the list by adding the RISC-V ARCHITECTURE maintainer entry
profile found in MAINTAINERS.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20200815115728.15128-1-lukas.bulwahn@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>