In some cases, especially when dealing with system and platform devices, a 
device's driver is known when the device is registered. We still want to add
the device to the driver's list and add it to the class.

This makes splits driver binding into probe() and attach(). If the device already
has a driver, we simply call attach(). Otherwise, we try to match it on the bus
and still call found_match().

This requires that all drivers that are referenced are registered beforehand.