media: cec: move cec_register_cec_notifier to cec-notifier.h

The cec_register_cec_notifier function was in media/cec.h, but it
has to be in cec-notifier.h.

While we are at it, also document it and add a stub function for when
the notifier is disabled or the CEC core code is unreachable.

Based on an earlier patch from Jose Abreu <Jose.Abreu@synopsys.com>.

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