intel_pstate: Add setting voltage value for baytrail P states.

Baytrail requires setting P state and voltage pairs when adjusting the
requested P state.  Add function for retrieving the valid voltage
values and modify *_set_pstate() functions to caluclate the
appropriate voltage for the requested P state.

Signed-off-by: Dirk Brandewie <dirk.j.brandewie@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>