media: staging/imx: of: Remove recursive graph walk

After moving to subdev notifiers, it's no longer necessary to recursively
walk the OF graph, because the subdev notifiers will discover and add
devices from the graph for us.

So the recursive of_parse_subdev() function is gone, replaced with
of_add_csi() which adds only the CSI port fwnodes to the imx-media
root notifier.

Signed-off-by: Steve Longerbeam <slongerbeam@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>