docs: xilinx: convert eemi.txt to eemi.rst

This is a very trivial conversion: adjust the title markup
and add a few literal block markups to produce a better
visual when parsed and avoid warnings.

As newer documents related to xilinx could be added in the future,
create a new index file for it.

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: Jonathan Corbet <corbet@lwn.net>