MAINTAINERS: add S390 MM section

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: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>