Alexander Gordeev and Gerald Schaefer are covering the whole s390 specific
memory management code. Reflect that by adding a new S390 MM section to
MAINTAINERS.

Also rename the S390 section to S390 ARCHITECTURE to be a bit more precise.
Acked-by: NGerald Schaefer <gerald.schaefer@linux.ibm.com>
Acked-by: NChristian Borntraeger <borntraeger@linux.ibm.com>
Acked-by: NAlexander Gordeev <agordeev@linux.ibm.com>
Acked-by: NVasily Gorbik <gor@linux.ibm.com>
Signed-off-by: NHeiko Carstens <hca@linux.ibm.com>
Signed-off-by: NAlexander Gordeev <agordeev@linux.ibm.com>