x86/PCI: MMCONFIG: use a private structure rather than the ACPI MCFG one

This adds a struct pci_mmcfg_region with a little more information
than the struct acpi_mcfg_allocation used previously.  The acpi_mcfg
structure is defined by the spec, so we can't change it.

To begin with, struct pci_mmcfg_region is basically the same as the
ACPI MCFG version, but future patches will add more information.

Reviewed-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>