media: renesas: vsp1: Implement .link_validate() for video devices

media: renesas: vsp1: Implement .link_validate() for video devices

The v4l2_subdev_link_validate() helper prints a warning if the
.link_validate() operation is not implemented for video devices
connected to the subdevs. Implement the operation to silence the
warning.

Ideally validation of the link between the video device and the subdev
should be implemented in that operation. That would however break
userspace that does not configure formats on all video devices before
starting streaming. While this mode of operation may not be considered
valid by the V4L2 API specification (interpretation differ), it is
nonetheless supported by the vsp1 driver at the moment and used by at
least the vsp1 unit test suite, and possibly other userspace
applications. Removing it would be a regression.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>