Add missing storage key initialization when memory comes online.
Didn't hurt until now, since the kernel always uses access key 0.

However for debugging purposes we sometimes use a different key
and it might be good if everything is properly initialized...
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>