[media] soc-camera: make .clock_{start,stop} compulsory, .add / .remove optional

All existing soc-camera host drivers use .clock_start() and .clock_stop()
callbacks to activate and deactivate their camera interfaces, whereas
.add() and .remove() callbacks are usually dummy. Make the former two
compulsory and the latter two optional.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>