media: add V4L2_CID_UNIT_CELL_SIZE control

This control returns the unit cell size in nanometres. The struct provides
the width and the height in separated fields to take into consideration
asymmetric pixels and/or hardware binning.
This control is required for automatic calibration of sensors/cameras.

Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Ricardo Ribalda Delgado <ribalda@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>