watchdog/diag288: correctly register for system reset requests

watchdog/diag288: correctly register for system reset requests

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: Xu Wang <gesaint@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>