media: add V4L2_SUBDEV_FL_STREAMS

Add subdev flag V4L2_SUBDEV_FL_STREAMS. It is used to indicate that the
subdev supports the new API with multiplexed streams (routing, stream
configs).

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>