hw/arm/mps2-tz: Make SYSCLK frequency board-specific

The AN524 has a different SYSCLK frequency from the AN505 and AN521;
make the SYSCLK frequency a field in the MPS2TZMachineClass rather
than a compile-time constant so we can support the AN524.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210215115138.20465-2-peter.maydell@linaro.org