[media] doc-rst: linux_tv CEC part, DocBook to reST migration

This is the reST migration of media's CEC part.  The migration is based
on media_tree's cec branch:

 https://git.linuxtv.org/media_tree.git



 c7169ad5 * cec media_tree/cec [media] DocBook/media: add CEC documentation

Signed-off-by: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>