The Freescale PCIe Root Complex shows up as a Processor class device, PowerPC
subclass, so the generic PCI code ignores it for a bridge. This adds support
for it.

As part of this, update the Freescale PCI hostbridge driver, to allow probing
beyond the root complex, instead of only allowing "proper" PCI-PCI bridges. An
alternative, which is #if 0'd out currently, is a hack to force the secondary
bus to 'primary plus 1' if a stable PCIe link is detected. This makes the
assumption that PCIe can never have more than one bus, but it's not as elegant
as the solution I proposed. However, the solution proposed may see false
positives, and has not been tested beyond the MPC8544 in the RouterBoard RB800.