device property: fix fwnode_graph_get_next_endpoint() documentation

Sync documentation with code.

Fixes: 07bb80d4 (device property: Add support for remote endpoints)
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>