PCI: Make pci_register_host_bridge() PCI core internal
With the introduction of pci_scan_root_bus_bridge() there is no need to
export pci_register_host_bridge() to other kernel subsystems other than the
PCI compilation unit that needs it.
Make pci_register_host_bridge() static to its compilation unit and convert
the existing drivers usage over to pci_scan_root_bus_bridge().