* Define a new static method DEVICE_IDENTIFY which is called to add device

* Define a new static method DEVICE_IDENTIFY which is called to add device
  instances to a parent bus.
* Define a new method BUS_ADD_CHILD which can be called from DEVICE_IDENTIFY
  to add new instances.
* Add a generic implementation of DEVICE_PROBE which calls DEVICE_IDENTIFY
  for each driver attached to the parent's devclass.
* Move the hint-based isa probe from the isa driver to a new isahint driver
  which can be shared between i386 and alpha.