MAINTAINERS: Add Documentation/cpu-freq/

I am told the cpufreq documentation updates should go to the PM list.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>