Add BCM2835 SOC MPHI (Message-based Parallel Host Interface)
emulation. It is very basic, only providing the FIQ interrupt
needed to allow the dwc-otg USB host controller driver in the
Raspbian kernel to function.

Signed-off-by: Paul Zimmerman <pauldzim@gmail.com>
Acked-by: Philippe Mathieu-Daude <f4bug@amsat.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200520235349.21215-2-pauldzim@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>