dm: core: Clarify behaviour of ofnode_name_eq()

This function is somewhat ambiguous, so expand the comments and add a
test for the undefined behaviour.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/dm/core.c

@@ -186,6 +186,8 @@ static int dm_test_compare_node_name(struct unit_test_state *uts)
 	ut_assert(ofnode_valid(node));
 	ut_assert(ofnode_name_eq(node, "mmio-bus"));
 
+	ut_assert(!ofnode_name_eq(node, "mmio-bus@0"));
+
 	return 0;
 }
 DM_TEST(dm_test_compare_node_name, UTF_SCAN_PDATA);