[media] v4l: Improve sub-device documentation for pad ops

Document that format related configuration is done through pad ops in case
the driver does use the media framework.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

Documentation/video4linux/v4l2-framework.txt

@@ -266,11 +266,16 @@ struct v4l2_subdev_video_ops {
 	...
 };
 
+struct v4l2_subdev_pad_ops {
+	...
+};
+
 struct v4l2_subdev_ops {
 	const struct v4l2_subdev_core_ops  *core;
 	const struct v4l2_subdev_tuner_ops *tuner;
 	const struct v4l2_subdev_audio_ops *audio;
 	const struct v4l2_subdev_video_ops *video;
+	const struct v4l2_subdev_pad_ops *video;
 };
 
 The core ops are common to all subdevs, the other categories are implemented
@@ -307,6 +312,10 @@ Don't forget to cleanup the media entity before the sub-device is destroyed:
 
 	media_entity_cleanup(&sd->entity);
 
+If the subdev driver intends to process video and integrate with the media
+framework, it must implement format related functionality using
+v4l2_subdev_pad_ops instead of v4l2_subdev_video_ops.
+
 A device (bridge) driver needs to register the v4l2_subdev with the
 v4l2_device: