of_graph: add of_graph_get_port_parent()

Linux kernel already has of_graph_get_remote_port_parent(),
but, sometimes we want to get own port parent.
This patch adds of_graph_get_port_parent()

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>