of: introduce of_graph_get_remote_node

The OF graph API leaves too much of the graph walking to clients when
in many cases the driver doesn't care about accessing the port or
endpoint nodes. The drivers typically just want the device connected via
a particular graph connection. of_graph_get_remote_node provides this
functionality.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>