Raw camera sensors connected to parallel or CSI-2 busses need to implement
V4L2_CID_LINK_FREQ control so receiver drivers can configure the hardware
accordingly.

Document this. Also say V4L2_CID_PIXEL_RATE is recommended for such
hardware.
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>