sampling_down_factor tunable is unused since commit
8e677ce8 (4 years ago).

This patch restores the original functionality and documents the
tunable.
Signed-off-by: NStratos Karafotis <stratosk@semaphore.gr>
Acked-by: NViresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>