docs: misc-devices/spear-pcie-gadget.txt: convert to ReST

- Use title/chapter markups;
- Use table markups;
- Mark literal blocks as such;
- Adjust indentation.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/4f8e129a93d04426ad7dd51f109725c48158a46d.1592203650.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>