media: admin-guide: cec.rst

Document administration details about CEC devices. This was formerly
documented in a cec-status.txt I kept on my website, but this really
belongs here as an admin guide.

Updated the original cec-status.txt, and converted it to .rst.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>