[media] V4L: Add v4l2_ctrl_subdev_subscribe_event() helper function

Add a v4l2 core helper function that can be used as the subdev
.subscribe_event handler. This allows to eliminate some boilerplate
from drivers that are only handling the control events.

Signed-off-by: Sylwester Nawrocki <sylvester.nawrocki@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>