ArmVirtPkg/FdtPciHostBridgeLib: add MMIO64 support
If the pci-host-ecam-generic DT node describes a 64-bit MMIO region,
account for it in the PCI_ROOT_BRIDGE description that we return to
the generic PciHostBridgeDxe implementation, which will be able to
allocate BARs from it without any further changes.