cpufreq: mediatek: add cleanups with the more generic naming

Since more MediaTek SoCs can be supported with the cpufreq driver and not
limited to MT8173, a couple of cleanups are done here with renaming those
functions and related structures with "mtk" instead of "mt8173".

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>