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>
Reviewed-by: Olivier Martin <Olivier.martin@arm.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16905 6f19259b-4bc3-4df7-8a09-765794883524

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