[media] v4l: Add V4L2_SEL_TGT_NATIVE_SIZE selection target

The V4L2_SEL_TGT_NATIVE_SIZE target is used to denote e.g. the size of a
sensor's pixel array.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>