cpufreq: mediatek: Use module_init and add module_exit

- Use module_init instead of device_initcall.
- Add a function for module_exit to unregister driver.

Signed-off-by: Jia-Wei Chang <jia-wei.chang@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>