MAINTAINERS: Add reviewer entry type and make me a s390 reviewer

As I missed some mails that introduced problems on s390 and their
fixes, this might help to broaden review coverage.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Suggested-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20180927112835.134812-1-frankja@linux.ibm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>