intel_idle: Add ->enter_freeze callbacks

Add an ->enter_freeze callback routine, intel_idle_freeze(), to
the intel_idle driver and point the ->enter_freeze callback
pointers of all of the driver's state objects to it.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>