docs: sparc: convert to ReST

Rename the sparc documentation files to ReST, add an
index for them and adjust in order to produce a nice html
output via the Sphinx build system.

There is an except from a document under oradax dir.
It doesn't seem to make much sense to convert this one
to ReST, so let's add it as an included document.

At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>