Pull of-graph helpers from Philipp Zabel:
of: Add of-graph helpers to loop over endpoints and find ports by id

This series converts of_graph_get_next_endpoint to decrement the refcount of
the passed prev parameter. This allows to add a for_each_endpoint_of_node
helper macro to loop over all endpoints in a device tree node.
The of_graph_get_port_by_id function is added to retrieve a port by its known
port id (contained in the reg property) from the device tree.