pinctrl: pinctrl.txt: standardize document format

Each text file under Documentation follows a different
format. Some doesn't even have titles!

Change its representation to follow the adopted standard,
using ReST markups for it to be parseable by Sphinx.

This document is almost following the standard stile.

There are only two things to adjust on it:

- promote the level of the document title;
- mark literal blocks as such.
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>
[Fix some indentations]
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>