We can use one MPC per SRAM bank, but we currently only wire the
IRQ from the first expansion MPC to the IRQ splitter. Fix that.

Fixes: bb75e16d ("hw/arm/iotkit: Wire up MPC interrupt lines")
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20201107193403.436146-2-f4bug@amsat.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>