introduce mmio_interface

This introduces mmio_interface object which contains a MemoryRegion
and can be hotplugged/hotunplugged.

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>