[media] cec: add proper support for CDC-Only CEC devices

CDC-Only CEC devices are CEC devices that can only handle CDC messages,
all other messages are ignored.

Add a flag to signal that this is a CDC-Only device and act accordingly.

Also add helper functions to identify if a CEC device is configured as a
CDC-Only device, a second TV, a switch or a processor, since these variations
cannot be determined by the logical address alone.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>