Documentation: v4l: Document rotation and orientation for sensor drivers

Document how rotation and orientation should be taken into account in
writing camera sensor drivers.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>