test: create dedicated fdt node for ofnode_for_each_prop test

Property count may change in /buttons node, if more button tests added,
and this will break ofnode_for_each_prop.
Add separate node for mentioned test.

Signed-off-by: Dzmitry Sankouski <dsankouski@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/ofnode.c b/test/dm/ofnode.c
index 8077aff..473a8ce 100644
--- a/test/dm/ofnode.c
+++ b/test/dm/ofnode.c
@@ -1046,7 +1046,7 @@
 	struct ofprop prop;
 	int count;
 
-	node = ofnode_path("/buttons");
+	node = ofnode_path("/ofnode-foreach");
 	count = 0;
 
 	/* we expect "compatible" for each node */