qdev: add MemoryRegion property

We need to pass a pointer to a MemoryRegion for mmio_interface.
So this just adds that.

Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>