s390x/pci: PCI multibus bridge handling

When the hotplug handler detects a PCI bridge, the secondary bus has
been initialized by the core PCI code. We give the secondary bus the
bridge name and associate to it the IOMMU handling and
hotplug/hotunplug callbacks.

Signed-off-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
