MAINTAINERS: downgrade myself to Reviewer for s390

Now that Alexander Gordeev has volunteered to be a co-maintainer for
s390, I can act as a reviewer instead of being a maintainer for s390.
With Alexander, Heiko, and Vasily we are in really good shape.
I will continue to act as the maintainer for KVM on s390 together with
Janosch.

Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>