[S390] pm: netiucv power management callbacks.

Patch establishes a dummy netiucv device to make sure iucv is notified
about suspend/resume even if netiucv is the only loaded iucv-exploting
module without any real net_device defined.

The PM freeze callback closes all open netiucv connections. Thus the
corresponding iucv path is removed.
The PM thaw/restore callback re-opens previously closed netiucv
connections.

Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>