Introduce pci_mmcfg_arch_map()/pci_mmcfg_arch_unmap(), which will be used
when supporting PCI root bridge hotplug.
Reviewed-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Jiang Liu <liuj97@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>