Implement a model of the Message Handling Unit (MHU) found in
the Arm SSE-200. This is a simple device which just contains
some registers which allow the two cores of the SSE-200
to raise interrupts on each other.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190219125808.25174-2-peter.maydell@linaro.org