powerpc/booke: Simplify the MPC85XX PCIe root complex driver

Summary:
Due to bugs in the enumeration code, fsl_pcib_init() was not configuring
sub-bridges properly, so devices hanging off a separate bridge would not
be found. Since the generic PCI code already supports probing child
buses, just delete this code and initialize only the device itself,
letting the generic code handle all the additional probing and
initializing.

This also deletes setup for some PCI peripherals found on some MPC85XX
evaluation boards. The code can be resurrected if needed, but overly
complicated this code in the first place.

Reviewed by: bdragon
Differential Revision: https://reviews.freebsd.org/D22050