PM / QoS: Add support for MIN/MAX frequency constraints

This patch introduces the min-frequency and max-frequency device
constraints, which will be used by the cpufreq core to begin with.

Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>