cpufreq / Documentation: Update cpufreq MAINTAINERS entry

Update the MAINTAINERS entry for cpufreq by making it clear that it
is not just drivers and adding current documentation records to it.

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