device property: export set_secondary_fwnode() to modules

Some drivers when compiled as modules may need to set secondary firmware node.
Export set_secondary_fwnode() to make it possible without code duplication.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Rafael J. Wysocki <rjw@rjwysocki.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>

drivers/base/core.c

@@ -3891,6 +3891,7 @@ void set_secondary_fwnode(struct device *dev, struct fwnode_handle *fwnode)
 	else
 		dev->fwnode = fwnode;
 }
+EXPORT_SYMBOL_GPL(set_secondary_fwnode);
 
 /**
  * device_set_of_node_from_dev - reuse device-tree node of another device