docs: sh: convert register-banks.txt to ReST

- Add a SPDX header;
- Adjust document title to follow ReST style;
- Add blank lines to make ReST markup happy
- Add it to sh/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/adf117cf1edd7f43cb839ff2800f4315dfbcce13.1592203650.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>