Add add() callback to struct bus_type.

This is so other entities in the kernel can inform the bus about devices
that it either doesn't know about yet (like when using the firmware to 
enumerate the system) or that it wouldn't otherwise find (for the same 
reason, or because they're weird legacy devices that won't be probed for).

include/linux/device.h

@@ -66,6 +66,7 @@ struct bus_type {
 	struct driver_dir_entry	driver_dir;
 
 	int		(*match)(struct device * dev, struct device_driver * drv);
+	struct device * (*add)	(struct device * parent, char * bus_id);
 };