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>