media: v4l: async: Allow binding notifiers to sub-devices

Registering a notifier has required the knowledge of struct v4l2_device
for the reason that sub-devices generally are registered to the
v4l2_device (as well as the media device, also available through
v4l2_device).

This information is not available for sub-device drivers at probe time.

What this patch does is that it allows registering notifiers without
having v4l2_device around. Instead the sub-device pointer is stored in the
notifier. Once the sub-device of the driver that registered the notifier
is registered, the notifier will gain the knowledge of the v4l2_device,
and the binding of async sub-devices from the sub-device driver's notifier
may proceed.

The complete callback of the root notifier will be called only when the
v4l2_device is available and no notifier has pending sub-devices to bind.
No complete callbacks are supported for sub-device notifiers.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>