cpufreq: mt8173: Mark mt8173_cpufreq_driver_init as __init

This function is only called once at boot by device_initcall(), so mark
it as __init.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>