cpufreq: stats: Use lock by stat to replace global spin lock

Stats is updated by each policy, using the lock by stat can
reduce the contention.

Signed-off-by: Kyle Lin <linkyle0915@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>