of: property: add missing kerneldoc for of_graph_get_endpoint_count()

of_graph_get_endpoint_count() doesn't have kerneldoc. Add it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://lore.kernel.org/r/87r0hrcvzk.wl-kuninori.morimoto.gx@renesas.com


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