Add a common bhnd_pci driver shared by both bhnd_pcib and bhnd_pci_hostb

[landon_landonf.org]

Summary

This extracts common code from bhndb_pci, bhnd_pcib, and bhnd_pci_hostb into a
simpler shared bhnd_pci base driver, and should enable SoC-side implementation
of bhnd_pcib root complex support.

Depends on D5759

[adrian]

This did'nt cleanly apply to -HEAD. Please update and test what's in HEAD so we can get a working patch :)

[landon_landonf.org]

rebased against HEAD