ACPI / CPPC: Add support for guaranteed performance

The Continuous Performance Control package may contain an optional
guaranteed performance field.

Add support to read guaranteed performance from _CPC.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>