[media] cec.h: add cec header

This header contains the CEC public API. Since the CEC framework will
initially be part of staging this header is kept out of include/uapi for
the time being until the CEC framework will be moved out of staging.
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>