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 <[e-mail]>
Acked-by: Heiko Carstens <[e-mail]>
Signed-off-by: Vasily Gorbik <[e-mail]>

MAINTAINERS

···
 S390
 M:	Heiko Carstens <[e-mail]>
 M:	Vasily Gorbik <[e-mail]>
-M:	Christian Borntraeger <[e-mail]>
 M:	Alexander Gordeev <[e-mail]>
+R:	Christian Borntraeger <[e-mail]>
 R:	Sven Schnelle <[e-mail]>
 L:	[e-mail]
 S:	Supported