[media] v4l: async: make v4l2 coexist with devicetree nodes in a dt overlay

In asds configured with V4L2_ASYNC_MATCH_OF, the v4l2 subdev can be
part of a devicetree overlay, for example:

&media_bridge {
	...
	my_port: port@0 {
		#address-cells = <1>;
		#size-cells = <0>;
		reg = <0>;
		ep: endpoint@0 {
			remote-endpoint = <&camera0>;
		};
	};
};

/ {
	fragment@0 {
		target = <&i2c0>;
		__overlay__ {
			my_cam {
				compatible = "foo,bar";
				port {
					camera0: endpoint {
						remote-endpoint = <&my_port>;
						...
					};
				};
			};
		};
	};
};

Each time the overlay is applied, its of_node pointer will be
different.  We are not interested in matching the pointer, what we
want to match is that the path is the one we are expecting.  Change to
use of_node_cmp() so that we continue matching after the overlay has
been removed and reapplied.

Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Javi Merino <javi.merino@kernel.org>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>