media: staging: max96712: Add basic support for MAX96712 GMSL2 deserializer

Add basic support for Maxim MAX96712 quad GMSL2 deserializers. The
driver is capable of powering on the device and configuring the MIPI
CSI-2 bus in a DPHY 4-lane configuration as well as operating the
internal VTG (Video Timing Generator) and VPG (Video Pattern Generator).

Using these features the driver is able to act as a 1080p @ 30 fps V4L2
video source. Producing either a checkerboard or gradient pattern on the
CSI-2 bus, selectable thru a V4L2 control.

While the driver is useful as-is and have been used to prove the correct
operation of the MAX96712 itself and "downstream" devices using the
MAX96712 as a video source there are a lot of features missing. Most
notably the ability to operate the GMSL bus.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>