Expand the Freescale PCIe root complex driver with the ofw_pcib_pci

The interrupt map wasn't being allocated properly, preventing IRQs from being
allocated to children of the PCIe bus. Fix this by cloning the ofw_pcib_pci
code, which handles all cases -- device tree and probed.

In the future this may become a subclass of the ofw_pcib_pci driver, but as
that's not an exported class, it's cloned for now.

MFC after: 3 weeks