[media] DocBook/media: add CEC documentation

Add DocBook documentation for the CEC API.

Signed-off-by: Hans Verkuil <hansverk@cisco.com>
[k.debski@samsung.com: add documentation for passthrough mode]
[k.debski@samsung.com: minor fixes and change of reserved field sizes]
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>