device property: fwnode_property_get_reference_args allows NULL args now

device property: fwnode_property_get_reference_args allows NULL args now

All three fwnode_property_get_reference_args() implemantations now allow
args argument to be NULL. Document this.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/r/20231109101010.1329587-4-sakari.ailus@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>