of: Warn if of_graph_parse_endpoint is called with the root node

If of_graph_parse_endpoint is given a parentless node instead of an
endpoint node, it is clearly a bug.
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>

drivers/of/base.c

@@ -1996,6 +1996,9 @@ int of_graph_parse_endpoint(const struct device_node *node,
 {
 	struct device_node *port_node = of_get_parent(node);
 
+	WARN_ONCE(!port_node, "%s(): endpoint %s has no parent node\n",
+		  __func__, node->full_name);
+
 	memset(endpoint, 0, sizeof(*endpoint));
 
 	endpoint->local_node = node;