[bhnd] Add a common bhnd_pci driver shared by both bhnd_pcib and bhnd_pci_hostb

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.

Submitted by: Landon Fuller <landonf@landonf.org>
Differential Revision: https://reviews.freebsd.org/D5763