dt: cpufreq: st: Provide bindings for ST's CPUFreq implementation

Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>