media: subdev: add v4l2_subdev_routing_validate() helper

Add a v4l2_subdev_routing_validate() helper for verifying routing for
common cases like only allowing non-overlapping 1-to-1 streams.

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