[media] pxa_camera: Add (un)subscribe_event ioctl

The v4l2-compliance complains about nonexistent vidioc_subscribe_event
and vidioc_unsubscribe_event calls. Add them to fix the complaints.

Signed-off-by: Petr Cvek <petr.cvek@tul.cz>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>