s390/configs: enable CONFIG_PCI_IOV

All major distributions ship with CONFIG_PCI_IOV=y so let us enable it
for our defconfigs as well.

Note also that since commit e5794cf1 ("s390/pci: create links
between PFs and VFs") we enabled proper linking between PFs and their
associated VFs so with this commit and its fixes applied we can fully
support handling SR-IOV enabled PFs.

Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>