powerpc/mpc85xx: Make Freescale PCI bridge driver a subclass of ofw_pcib_pci

This driver was already 99% identical to the ofw_pcib_pci driver, except for
the attachment. Since ofw_pcib_pci is already a subclass of pcib, this
creates a private declaration of that class, to use for the base class for
this driver.

At some point in the future, ofw_pcib_pci_driver should probably be exported
to a header, so we're not tracking the softc struct contents, but for now,
since there's only this one other driver, it's not a pressing issue.