media: staging/imx: TODO: Remove one assumption about OF graph parsing

  decided whether this feature is useful enough to make it generally

  available by exporting to v4l2-core.

- The OF graph is walked at probe time to form the list of fwnodes to

  be passed to v4l2_async_notifier_register(), starting from the IPU

  CSI ports. And after all async subdevices have been bound,

  v4l2_fwnode_parse_link() is used to form the media links between

  the entities discovered by walking the OF graph.

- After all async subdevices have been bound, v4l2_fwnode_parse_link()

  is used to form the media links between the devices discovered in

  the OF graph.

  While this approach allows support for arbitrary OF graphs, there

  are some assumptions for this to work:

  1. All port parent nodes reachable in the graph from the IPU CSI

     ports bind to V4L2 async subdevice drivers.

     If a device has mixed-use ports such as video plus audio, the

     endpoints from the audio ports are followed to devices that must

     bind to V4L2 subdevice drivers, and not for example, to an ALSA

     driver or a non-V4L2 media driver. If the device were bound to

     such a driver, imx-media would never get an async completion

     notification because the device fwnode was added to the async

     list, but the driver does not interface with the V4L2 async

     framework.

  2. Every port reachable in the graph is treated as a media pad,

     owned by the V4L2 subdevice that is bound to the port's parent.

  1. If a port owned by a device in the graph has endpoint nodes, the

     port is treated as a media pad.

     This presents problems for devices that don't make this port = pad

     assumption. Examples are SMIAPP compatible cameras which define only

     possible long-term solution is to implement a subdev API that

     maps a port id to a media pad index.

  3. Every endpoint of a port reachable in the graph is treated as

     a media link, between V4L2 subdevices that are bound to the

     port parents of the local and remote endpoints.

  2. Every endpoint of a port owned by a device in the graph is treated

     as a media link.

     Which means a port must not contain mixed-use endpoints, they

     must all refer to media links between V4L2 subdevices.