s390: add Sven Schnelle as reviewer

Sven Schnelle will help reviewing s390 architecture code.
Acked-by: NSven Schnelle <svens@linux.ibm.com>
Acked-by: NAlexander Gordeev <agordeev@linux.ibm.com>
Acked-by: NVasily Gorbik <gor@linux.ibm.com>
Acked-by: NChristian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: NHeiko Carstens <hca@linux.ibm.com>

MAINTAINERS

@@ -16611,6 +16611,7 @@ 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>
+R:	Sven Schnelle <svens@linux.ibm.com>
 L:	linux-s390@vger.kernel.org
 S:	Supported
 W:	http://www.ibm.com/developerworks/linux/linux390/