hw/arm/mps2-tz: Stub out USB controller for mps3-an524

hw/arm/mps2-tz: Stub out USB controller for mps3-an524

The AN524 has a USB controller (an ISP1763); we don't have a model of
it but we should provide a stub "unimplemented-device" for it.  This
is slightly complicated because the USB controller shares a PPC port
with the ethernet controller.

Implement a make_* function which provides creates a container
MemoryRegion with both the ethernet controller and an
unimplemented-device stub for the USB controller.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Message-id: 20210215115138[email protected]