power: add get/set min/max scaling frequencies API

power: add get/set min/max scaling frequencies API

Add new get/set API to allow the user or application to set the minimum
and maximum frequencies to use when scaling.
Previously, the frequency range was determined by the HW capabilities of
the CPU. With this new API, the user or application can constrain this
if required.

Signed-off-by: Kevin Laatz <kevin.laatz@intel.com>
Acked-by: Ray Kinsella <mdr@ashroe.eu>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>
Tested-by: David Hunt <david.hunt@intel.com>