[bhnd] Finish bhnd(4) PCI/PCIe-G1 hostb support.

Now that we've got access to SPROM and can access board identification,
this implements all known remaining hardware work-arounds for the bhnd(4)
PCI and PCIe-G1 cores operating endpoint mode.

Additionally, this adds an initial set of skeleton PCIe-G2 hostb and pcib
drivers, required by fullmac and newer softmac devices.

Submitted by: Landon Fuller <landonf@landonf.org>
Differential Revision: https://reviews.freebsd.org/D6377