merge_dtbs: ignore not matching DTBOs when finding symbols for a fixup

	for dt in devicetrees:

		for symbol in dt.list_props('/__symbols__'):

			symbol_map.setdefault(symbol, []).append(dt.filename)

			symbol_map.setdefault(symbol, []).append(dt)

	for dt in devicetrees:

		graph[dt.filename] = set()

			if fixup not in symbol_map:

				continue

			graph[dt.filename].update(symbol_map[fixup])

			for symbol_dt in symbol_map[fixup]:

				if dt == symbol_dt:

					assert not len(graph[dt.filename])

					graph[dt.filename].add(symbol_dt)

	return graph