cpufreq: stats: New sysfs attribute for clearing statistics

Allow CPUfreq statistics to be cleared by writing anything to
/sys/.../cpufreq/stats/reset.

Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>