x86/intel_idle: add Gemini Lake support

Gemini Lake uses the same C-states as Broxton and also uses the
IRTL MSR's to determine maximum C-state latency.

Signed-off-by: David E. Box <david.e.box@linux.intel.com>
Acked-by: Len Brown <len.brown@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>