cpufreq: bmips-cpufreq: 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: Zhang Jianhua <chris.zjh@huawei.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>