of: property: use new of_graph functions

Current of_graph_get_next_endpoint() can be replaced by using
new of_graph_get_next_port().

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87iktib5t0.wl-kuninori.morimoto.gx@renesas.com


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>