media: sun6i-csi: Add support for hooking to the isp devices

In order to use the isp and csi together, both devices need to be
parented to the same v4l2 and media devices. We use the isp as
top-level device and let the csi code hook to its v4l2 and media
devices when async subdev registration takes place.

As a result v4l2/media device setup is only called when the ISP
is missing and the capture device is registered after the devices
are hooked. The bridge subdev and its notifier are registered
without any device when the ISP is available. Top-level pointers
for the devices are introduced to either redirect to the hooked ones
(isp available) or the registered ones (isp missing).

Also keep track of whether the capture node was setup or not to
avoid cleaning up resources when it wasn't.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>