MAINTAINERS: add/replace backups for some s390 areas

MAINTAINERS: add/replace backups for some s390 areas

S390 PCI currently has no backup, add one.  Add an additional backup
for vfio-ccw and refresh the backup for vfio-ap.

Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
Acked-by: Jason J. Herne <jjherne@linux.ibm.com>
Acked-by: Eric Farman <farman@linux.ibm.com>
Acked-by: Pierre Morel <pmorel@linux.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Halil Pasic <pasic@linux.ibm.com>
Message-Id: <1616680509-8339-1-git-send-email-mjrosato@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>