ArmPkg: implement CpuIo2 protocol driver specific for PCI

ArmPkg: implement CpuIo2 protocol driver specific for PCI

The CpuIo2 protocol is required by the generic PciHostBridgeDxe driver,
which relies on it to back its own I/O and MMIO operations.

Since ARM has no native I/O port equivalent, such accesses can only
originate from PCI drivers, and the PCI I/O space is translated to MMIO
in this case.

So we can implement this protocol using MMIO operations only, and take
the PCI I/O translation offset into account when performing I/O port
accesses.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>