x86/PCI: MMCONFIG: add resource to struct pci_mmcfg_region
This patch adds a resource and corresponding name to the MMCONFIG
structure. This makes allocation simpler (we can allocate the
resource and name at the same time we allocate the pci_mmcfg_region),
and gives us a way to hang onto the resource after inserting it.
This will be needed so we can release and free it when hot-removing
a host bridge.