cpuidle: Add cpumask to ease backporting

To make backporting a little easier provide a cpumask variable in the
driver struct - the core won't do anything with it but it's less code
to maintain.

Signed-off-by: Mark Brown <broonie@linaro.org>

include/linux/cpuidle.h

@@ -111,6 +111,7 @@ struct cpuidle_driver {
 	struct cpuidle_state	states[CPUIDLE_STATE_MAX];
 	int			state_count;
 	int			safe_state_index;
+	struct cpumask		*cpumask;
 };
 
 #ifdef CONFIG_CPU_IDLE