media: atmel: atmel-isc-base: expose white balance as v4l2 controls

This exposes the white balance configuration of the ISC as v4l2 controls
into userspace.
There are 8 controls available:
4 gain controls, sliders, for each of the BAYER components: R, B, GR, GB.
These gains are multipliers for each component, in format unsigned 0:4:9
with a default value of 512 (1.0 multiplier).
4 offset controls, sliders, for each of the BAYER components: R, B, GR, GB.
These offsets are added/substracted from each component, in format signed
1:12:0 with a default value of 0 (+/- 0)

To expose this to userspace, added 8 custom controls, in an auto cluster.

To summarize the functionality:
The auto cluster switch is the auto white balance control, and it works
like this:
AWB == 1: autowhitebalance is on, the do_white_balance button is inactive,
the gains/offsets are inactive, but volatile and readable.
Thus, the results of the whitebalance algorithm are available to userspace
to read at any time.
AWB == 0: autowhitebalance is off, cluster is in manual mode, user can
configure the gain/offsets directly. More than that, if the
do_white_balance button is pressed, the driver will perform
one-time-adjustment, (preferably with color checker card) and the userspace
can read again the new values.

With this feature, the userspace can save the coefficients and reinstall
them for example after reboot or reprobing the driver.

[hverkuil: fix checkpatch warning]
[hverkuil: minor spacing adjustments in the functionality description]

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>