Modernize mpc85xx PCI hostbridge driver.

Summary:

• Take advantage of NEW_PCIB to remove a lot of setup code.
• Fix some bugs related to multiple PCI bridges.

There's still room for more cleanup, and still some bugs leftover, but this
cleans up a lot.

Test Plan: Tested on P5020 board with IDT PCIe switch.

Differential Revision: https://reviews.freebsd.org/D4127