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>
diff --git a/test/dm/core.c b/test/dm/core.c
index c59ffc6..d40916e 100644
--- a/test/dm/core.c
+++ b/test/dm/core.c
@@ -186,6 +186,8 @@
 	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);