devicetree parsing does not error out on duplicate node names

[mbolivar-nordic]

Describe the bug

dtc errors out on duplicate node names:

$ cat foo.dts
/dts-v1/;

/ {
	blub: foo@1234 {
		reg = <1234 4>;
		bar = <1>;
	};

	blab: foo@1234 {
		reg = <1234 4>;
		baz = <2>;
	};
};

$ dtc -Odts foo.dts
foo.dts:8.17-10.4: ERROR (duplicate_node_names): /foo@1234: Duplicate node name
ERROR: Input tree has errors, aborting (use -f to force output)

Zephyr's dtlib doesn't:

$ cat samples/hello_world/app.overlay
/ {
	blub: foo@1234 {
		reg = <1234 4>;
		bar = <1>;
	};

	blab: foo@1234 {
		reg = <1234 4>;
		baz = <2>;
	};
};

$ west build --cmake-only samples/hello_world # no errors

Expected behavior

Our DT parser should be equivalent to dtc and therefore should error out.

Impact

We accept garbage DT files that have duplicate nodes.

Additional context

Originally reported in https://github.com/zephyrproject-rtos/zephyr/pull/48949/files#r955645210