hw/arm/mps2-tz: Add IRQ infrastructure to support SSE-200

hw/arm/mps2-tz: Add IRQ infrastructure to support SSE-200

In preparation for adding support for the AN521 MPS2 image, we need
to handle wiring up the MPS2 device interrupt lines to both CPUs in
the SSE-200, rather than just the one that the IoTKit has.

Abstract out a "connect to the IoTKit interrupt line" function
and make it connect to a splitter which feeds both sets of inputs
for the SSE-200 case.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190121185118.18550-23-peter.maydell@linaro.org