s390/watchdog: enable KEEPALIVE for /dev/watchdog

There's no reason why we wouldn't want to be able to send a keep alive
message to /dev/watchdog (feed dog) for the s390 diag288 watchdog, so
let's enable the WDIOF_KEEPALIVEPING option.

Signed-off-by: Xu Wang <gesaint@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>