of_graph: add of_graph_get_endpoint_count()

OF graph want to count its endpoint number, same as
of_get_child_count(). This patch adds of_graph_get_endpoint_count()

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>