dummy: declare dummy devices as enumerated devices

Dummy device name is enumerated by the kernel, let user space be aware
of the naming scheme used by dummy devices:
(visible in /sys/class/net/<iface>/name_assign_type).

Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>