dtoc: make ScanTree recurse into subnodes

Previously, dtoc could only process the top-level nodes which led to
device nodes in hierarchical trees to be ignored. E.g. the mmc0 node
in the following example would be ignored, as only the soc node was
processed:

  / {
	soc {
		mmc0 {
			/* ... */
		};
	};
  };

This introduces a recursive helper method ScanNode, which is used by
ScanTree to recursively parse the entire tree hierarchy.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>