[media] v4l2-subdev: add registered_async subdev core operation

V4L2 sub-devices that are registered asynchronously have no way to know
when they have been registration with a V4L2 device but they might need
to take some action after this.

So let's add a callback that can be executed by the V4L2 async core to
allow sub-devices drivers to do any needed initialization that depends
on the registration.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>