[media] v4l: Image source control class

Add image source control class. This control class is intended to contain
low level controls which deal with control of the image capture process ---
the A/D converter in image sensors, for example.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>