MAINTAINERS: update KVM/s390 maintainers

As I have neither too much time nor access to the architecture
documentation anymore, let's switch my status from maintainer to
reviewer. Janosch will step in as second maintainer.

Acked-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>