cpufreq: mips: move cpufreq driver to drivers/cpufreq

This patch moves cpufreq driver of MIPS architecture to drivers/cpufreq.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>