ACPI: CPPC: add sysfs entry for guaranteed performance

Expose the CPPC guaranteed performance as reported by the platform through
GuaranteedPerformanceRegister.

The current value is already read in cppc_get_perf_caps() and stored in
struct cppc_perf_caps (to be used by the intel_pstate driver), so only the
attribute itself needs to be defined.

Signed-off-by: Petr Tesařík <ptesarik@suse.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>