ArmVirtPkg: add XenPlatformHasAcpiDtDxe

This driver produces the EDKII Platform Has ACPI and Platform Has Device
Tree protocols, exactly matching the current ACPI / DT exposure on Xen,
according to ARM vs. AARCH64. At this point it differs from the QEMU
driver PlatformHasAcpiDtDxe in that this one always installs the DT.

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