[media] cec.txt: add CEC framework documentation

Document the new HDMI CEC framework.

[k.debski@samsung.com: add DocBook documentation by Hans Verkuil, with
Signed-off-by: Kamil Debski <kamil@wypas.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>