docs: openrisc: convert to ReST and add to documentation body

Manually convert the two openRisc documents to ReST, adding them
to the Linux documentation body.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Stafford Horne <shorne@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>