Add a PCI bridge for the Freescale PCIe Root Complex

Summary:
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.

Reviewers: PowerPC, marcel, nwhitehorn

Reviewed By: nwhitehorn

Subscribers: imp

Differential Revision: https://reviews.freebsd.org/D2442

Relnotes: yes