MAINTAINERS: add Vasily Gorbik and Christian Borntraeger for s390

Add Vasily Gorbik and Christian Borntraeger as additional maintainers
for s390.

Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>