[S390] pm: vmwatchdog power management callbacks.

This patch implements suspend/hibernation for the vmwatchdog driver. The
pm_notifier_callchain is used to get control on PM events. Since watchdog
operation and suspend cannot work together in a reliable fashion, the open
flag is also used to prevent suspend and open from happening at the same
time.
The watchdog can also be active with no open file descriptor. This patch
adds another flag which is only changed in vmwdt_keep_alive and
vmwdt_disable.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>