doc: arch: Add document for RISC-V architecture

This patch adds a brief introduction to the RISC-V architecture and
the typical boot process used on a variety of RISC-V platforms.

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>