Driver Core: add nodename callbacks

This adds the nodename callback for struct class, struct device_type and
struct device, to allow drivers to send userspace hints on the device
name and subdirectory that should be used for it.

Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Jan Blunck <jblunck@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>