hw/arm/musca.c: Implement models of the Musca-A and -B1 boards

hw/arm/musca.c: Implement models of the Musca-A and -B1 boards

The Musca-A and Musca-B1 development boards are based on the
SSE-200 subsystem for embedded. Implement an initial skeleton
model of these boards, which are similar but not identical.

This commit creates the board model with the SSE and the IRQ
splitters to wire IRQs up to its two CPUs. As yet there
are no devices and no memory: these will be added later.

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