MAINTAINERS: add uv.c also to KVM/s390

Most changes in uv.c are related to KVM. Involve also the KVM team
regarding changes to uv.c.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>