media: ti: j721e-csi2rx: Use fwnode_get_named_child_node

media: ti: j721e-csi2rx: Use fwnode_get_named_child_node

Simplify notifier registration logic. Instead of first getting the
device node, get the fwnode of the child directly.

Reviewed-by: Devarsh Thakkar <devarsht@ti.com>
Tested-by: Yemike Abhilash Chandra <y-abhilashchandra@ti.com> (on SK-AM68)
Signed-off-by: Jai Luthra <jai.luthra@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>