cpuidle: declare cpuidle_dev in cpuidle.h

Declaring this allows drivers which need to initialise each struct
cpuidle_device at initialisation time to make use of the structures
already defined in cpuidle.c, rather than having to wastefully define
their own.
Signed-off-by: Paul Burton <paul.burton@imgtec.com>

include/linux/cpuidle.h

@@ -84,6 +84,7 @@ struct cpuidle_device {
 };
 
 DECLARE_PER_CPU(struct cpuidle_device *, cpuidle_devices);
+DECLARE_PER_CPU(struct cpuidle_device, cpuidle_dev);
 
 /**
  * cpuidle_get_last_residency - retrieves the last state's residency time