Driver core: add device_type to struct device

This allows us to add type specific attributes, uevent vars and
release funtions.

A subsystem can carry different types of devices like the "block"
subsys has disks and partitions. Both types create a different set
of attributes, but belong to the same subsystem.

This corresponds to the low level objects:
  kobject   -> device       (object/device data)
  kobj_type -> device_type  (type of object/device we are embedded in)
  kset      -> class/bus    (list of objects/devices of a subsystem)

Signed-off-by: Kay Sievers <kay.sievers@novell.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>