Currently the driver notifies internal subdevs if the signal is locked
or not. This information is also useful for userpace applications e.g. to
switch to another input device upon a signal lost event.

This commit adds the support for the userspace to subscribe to the
V4L2_EVENT_SOURCE_CHANGE and V4L2_EVENT_CTRL events.
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>