[media] adv7842: Add support for control event notifications

Allow userspace applications to subscribe to control change events. This
can e.g. be used to monitor the 5V detect control to be notified when a
source is connected or disconnected.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>