Expand the support for PCI-e memory mapped configuration space access.

Expand the support for PCI-e memory mapped configuration space access.

This defaults to off and must be explicitly
enabled by setting the loader tunable hw.pci.mcfg=1.
- Add support for the Intel 915GM chipsets by reading the BAR.
- Add parsing of the ACPI MCFG table to discover memory mapped configuration
  access on modern machines.
- For config requests to busses not listed in ACPI's min/max valid buses,
  fall back to using type #1 configuration access instead.
- Add a workaround for some K8 chipsets that do not expose all devices on
  bus 0 via MCFG and fall back to type #1 for those devices instead.

Obtained-from: FreeBSD