MAINTAINERS: add Alexander Gordeev as maintainer for s390

Change Alexander Gordeev's status so he is maintainer
instead of reviewer for s390.

Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

Author: hcahca

MAINTAINERS

@@ -16818,7 +16818,7 @@ S390
 M:	Heiko Carstens <hca@linux.ibm.com>
 M:	Vasily Gorbik <gor@linux.ibm.com>
 M:	Christian Borntraeger <borntraeger@linux.ibm.com>
-R:	Alexander Gordeev <agordeev@linux.ibm.com>
+M:	Alexander Gordeev <agordeev@linux.ibm.com>
 R:	Sven Schnelle <svens@linux.ibm.com>
 L:	linux-s390@vger.kernel.org
 S:	Supported