ArmVirtualizationPkg/PciHostBridgeDxe: allocate IO BARs top-down
    
    Currently we allocate IO BARs bottom-up in the
    EfiPciHostBridgeAllocateResources phase of the enumeration.
    
    > GCD:AddIoSpace(Base=0000000000000000,Length=0000000000010000)
    >   GcdIoType    = I/O
    >   Status = Success
    > GCDIoType  Range
    > ========== =================================
    > I/O        0000000000000000-000000000000FFFF
    
    Because the IO aperture is based at zero, the first allocation happens to
    get the zero address. However, a zero address for a PCI BAR is considered
    unmapped; see eg.:
    
    - <http://www.pcisig.com/reflector/msg00459.html>,
    
    - the (new_addr == 0) part in QEMU, pci_bar_address() [hw/pci/pci.c]:
    
        new_addr = pci_get_long(d->config + bar) & ~(size - 1);
        last_addr = new_addr + size - 1;
        /* Check if 32 bit BAR wraps around explicitly.
         * TODO: make priorities correct and remove this work around.
         */
        if (last_addr <= new_addr || new_addr == 0 || last_addr >= UINT32_MAX)
        {
            return PCI_BAR_UNMAPPED;
        }
    
    We can avoid this problem by allocating top-down in the IO aperture.
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>