arm: Remove system_clock_scale global

arm: Remove system_clock_scale global

All the devices that used to use system_clock_scale have now been
converted to use Clock inputs instead, so the global is no longer
needed; remove it and all the code that sets it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210812093356.1946-26-peter.maydell@linaro.org