atm: propagate signal changes via notifier

Add notifier chain for changes in atm_dev.

Clients like br2684 will call register_atmdevice_notifier() to be notified of
changes. Drivers will call atm_dev_signal_change() to notify clients like
br2684 of the change.

On DSL and ATM devices it's usefull to have a know if you have a carrier
signal. netdevice LOWER_UP changes can be propagated to userspace via netlink
monitor.

Signed-off-by: Karl Hiramoto <karl@hiramoto.org>
Signed-off-by: David S. Miller <davem@davemloft.net>