cpufreq: st: Provide runtime initialised driver for ST's platforms

The bootloader is charged with the responsibility to provide platform
specific Dynamic Voltage and Frequency Scaling (DVFS) information via
Device Tree.  This driver takes the supplied configuration and
registers it with the new generic OPP framework, to then be used with
CPUFreq.

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>