MAINTAINERS: Replace one of the maintainers for s390/pci

MAINTAINERS: Replace one of the maintainers for s390/pci

Add myself as co-maintainer for s390/pci, replacing Gerald Schaefer who
has moved his focus to s390/mm. Thank you Gerald!

Signed-off-by: Gerd Bayer <gbayer@linux.ibm.com>
Acked-by: Niklas Schnelle <schnelle@linux.ibm.com>
Acked-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>