MAINTAINERS: add backups for s390 vfio drivers

Add a backup for s390 vfio-pci, an additional backup for vfio-ccw
and replace the backup for vfio-ap as Pierre is focusing on other
areas.

Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
Acked-by: Eric Farman <farman@linux.ibm.com>
Acked-by: Halil Pasic <pasic@linux.ibm.com>
Acked-by: Pierre Morel <pmorel@linux.ibm.com>
Acked-by: Jason J. Herne <jjherne@linux.ibm.com>
Link: https://lore.kernel.org/r/1616679712-7139-1-git-send-email-mjrosato@linux.ibm.com
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>