[media] cec: rename MEDIA_CEC_NOTIFIER to CEC_NOTIFIER

This config option is strictly speaking independent of the
media subsystem since it can be used by drm as well.

Besides, it looks odd when drivers select CEC_CORE and
MEDIA_CEC_NOTIFIER, that's inconsistent naming.

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