The diag288 watchdog is no sysbus device, therefore it doesn't get
triggered on resets automatically using dc->reset.

Let's register the reset handler manually, so we get correctly notified
again when a system reset was requested. Also reset the watchdog on
subsystem resets that don't trigger a full system reset.
Signed-off-by: NXu Wang <gesaint@linux.vnet.ibm.com>
Reviewed-by: NDavid Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: NCornelia Huck <cornelia.huck@de.ibm.com>
Tested-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: NAndreas Färber <afaerber@suse.de>