of: Add of_graph_get_port_by_id function

This patch adds a function to get a port device tree node by port id,
or reg property value.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
diff --git a/include/linux/of_graph.h b/include/linux/of_graph.h
index e43442e..3c1c95a 100644
--- a/include/linux/of_graph.h
+++ b/include/linux/of_graph.h
@@ -40,6 +40,7 @@
 #ifdef CONFIG_OF
 int of_graph_parse_endpoint(const struct device_node *node,
 				struct of_endpoint *endpoint);
+struct device_node *of_graph_get_port_by_id(struct device_node *node, u32 id);
 struct device_node *of_graph_get_next_endpoint(const struct device_node *parent,
 					struct device_node *previous);
 struct device_node *of_graph_get_remote_port_parent(
@@ -53,6 +54,12 @@
 	return -ENOSYS;
 }
 
+static inline struct device_node *of_graph_get_port_by_id(
+					struct device_node *node, u32 id)
+{
+	return NULL;
+}
+
 static inline struct device_node *of_graph_get_next_endpoint(
 					const struct device_node *parent,
 					struct device_node *previous)