Add V4L2_CID_DIGITAL_GAIN to control explicitly digital gain.

We already have analogue gain control which the digital gain control
complements. Typically higher quality images are obtained using analogue
gain only as the digital gain does not add information to the image
(rather it may remove it).
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>