hw/arm/xlnx-versal: Add unimplemented APU mmio

hw/arm/xlnx-versal: Add unimplemented APU mmio

Add unimplemented APU mmio region to xlnx-versal for booting
bare-metal guests built with standalone bsp, which access the
region from one of the following places:
  https://github.com/Xilinx/embeddedsw/blob/release-2020.2/lib/bsp/standalone/src/arm/ARMv8/64bit/armclang/boot.S#L139
  https://github.com/Xilinx/embeddedsw/blob/release-2020.2/lib/bsp/standalone/src/arm/ARMv8/64bit/gcc/boot.S#L183

Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Tong Ho <tong.ho@xilinx.com>
Message-id: 20210823173818.201259-2-tong.ho@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>