hw/arm/msf2-soc: Wire up refclk

Wire up the refclk for the msf2 SoC.  This SoC runs the refclk at a
frequency which is programmably either /4, /8, /16 or /32 of the main
CPU clock.  We don't currently model the register which allows the
guest to set the divisor, so implement the refclk as a fixed /32 of
the CPU clock (which is the value of the divisor at reset).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Damien Hedde <damien.hedde@greensocs.com>
Message-id: 20210812093356.1946-21-peter.maydell@linaro.org