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>

MAINTAINERS

@@ -7692,8 +7692,9 @@ F:	arch/powerpc/kernel/kvm*
 
 KERNEL VIRTUAL MACHINE for s390 (KVM/s390)
 M:	Christian Borntraeger <borntraeger@de.ibm.com>
-M:	Cornelia Huck <cohuck@redhat.com>
+M:	Janosch Frank <frankja@linux.vnet.ibm.com>
 R:	David Hildenbrand <david@redhat.com>
+R:	Cornelia Huck <cohuck@redhat.com>
 L:	linux-s390@vger.kernel.org
 W:	http://www.ibm.com/developerworks/linux/linux390/
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/kvms390/linux.git