intel_pstate: Add support for Baytrail turbo P states

A documentation update exposed the existance of the turbo ratio
register. Update baytrail support to use the turbo range.

Signed-off-by: Dirk Brandewie <dirk.j.brandewie@intel.com>
Cc: 3.13+ <stable@vger.kernel.org> # 3.13+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>