media: uapi: Document which mbus format fields are valid for metadata

Now that metadata mbus formats have been added, it is necessary to define
which fields in struct v4l2_mbus_format are applicable to them (not many).

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>