[media] v4l: Add input and output capability flags for native size setting

Add input and output capability flags for setting native size of the device,
and document them.

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>