intel_idle: allow idle states to be freeze-mode specific

intel_idle uses a NULL "enter" field in a cpuidle state
to recognize the invalid entry terminating a variable-length array.

Linux-4.0 added support for the system-wide "freeze" state
in cpuidle drivers via the new "enter_freeze" field.

The natural way to expose a deep idle state for freeze,
but not for run-time idle is to supply "enter_freeze" without "enter";
so we update the driver to accept such states.

Signed-off-by: Len Brown <len.brown@intel.com>