media: v4l2-subdev: Document that routing support depends on streams

Routing support, through the subdev .set_routing() operation, requires
the subdev to support streams. This is however not clearly documented
anywhere. Fix it by expanding the operation's documentation to indicate
that subdevs must set the V4L2_SUBDEV_FL_STREAMS flag.

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