DMA-attributes.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 file is almost on the standard format. All it needs
to be parsed by Sphinx is to fix its main title and add
a few blank lines.

Yet, it is worth to replace NOTE: by the corresponding
ReST markup.

While here, remove the extra "---" from a subtitle.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>