We add a new control V4L2_CID_NOTIFY_GAINS which allows the sensor to
be notified what gains will be applied to the different colour
channels by subsequent processing (such as by an ISP), even though the
sensor will not apply any of these gains itself.

For Bayer sensors this will be an array control taking 4 values which
are the 4 gains arranged in the fixed order B, Gb, Gr and R,
irrespective of the exact Bayer order of the sensor itself. The use of
an array makes it straightforward to extend this control to non-Bayer
sensors (for example, sensors with an RGBW pattern) in future.

The units are in all cases linear with the default value indicating a
gain of exactly 1.0. For example, if the default value were reported as
128 then the value 192 would represent a gain of exactly 1.5.
Signed-off-by: David Plowman <david.plowman@raspberrypi.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>