MAINTAINERS: Update s390-iommu driver maintainer information

The s390 DMA API conversion changes currently under review will extend
the use of the s390-iommu driver to the DMA API. With s390's mandatory
use of an IOMMU this means all DMA for PCI devices will then use the
s390-iommu driver. With this in mind and considering my involvement in
these changes it makes sense to reflect this increased interdependence
in the maintainer structure. Thus add myself as first maintainer and
move Gerald to reviewer status.
Reviewed-by: NGerald Schaefer <gerald.schaefer@linux.ibm.com>
Reviewed-by: NMatthew Rosato <mjrosato@linux.ibm.com>
Acked-by: NPeter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: NNiklas Schnelle <schnelle@linux.ibm.com>
Link: https://lore.kernel.org/r/20230221161043.37065-1-schnelle@linux.ibm.comSigned-off-by: NJoerg Roedel <jroedel@suse.de>

MAINTAINERS

@@ -18291,8 +18291,9 @@ F:	drivers/s390/block/dasd*
 F:	include/linux/dasd_mod.h
 
 S390 IOMMU (PCI)
+M:	Niklas Schnelle <schnelle@linux.ibm.com>
 M:	Matthew Rosato <mjrosato@linux.ibm.com>
-M:	Gerald Schaefer <gerald.schaefer@linux.ibm.com>
+R:	Gerald Schaefer <gerald.schaefer@linux.ibm.com>
 L:	linux-s390@vger.kernel.org
 S:	Supported
 F:	drivers/iommu/s390-iommu.c