of: Add for_each_endpoint_of_node helper macro

Note that while of_graph_get_next_endpoint decrements the reference count
of the child node passed to it, of_node_put(child) still has to be called
manually when breaking out of the loop.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>