cpuidle: dt: Support hierarchical CPU idle states

Currently CPU's idle states are represented using the flattened model.
Let's add support for the hierarchical layout, via converting to use
of_get_cpu_state_node().

Suggested-by: Sudeep Holla <[e-mail]>
Signed-off-by: Lina Iyer <[e-mail]>
Reviewed-by: Daniel Lezcano <[e-mail]>
Co-developed-by: Ulf Hansson <[e-mail]>
Signed-off-by: Ulf Hansson <[e-mail]>
Reviewed-by: Sudeep Holla <[e-mail]>
Acked-by: Rafael J. Wysocki <[e-mail]>

drivers/cpuidle/dt_idle_states.c

···
 	for (cpu = cpumask_next(cpumask_first(cpumask), cpumask);
 	     cpu < nr_cpu_ids; cpu = cpumask_next(cpu, cpumask)) {
 		cpu_node = of_cpu_device_node_get(cpu);
-		curr_state_node = of_parse_phandle(cpu_node, "cpu-idle-states",
-						   idx);
+		curr_state_node = of_get_cpu_state_node(cpu_node, idx);
 		if (state_node != curr_state_node)
 			valid = false;
 
···
 	cpu_node = of_cpu_device_node_get(cpumask_first(cpumask));
 
 	for (i = 0; ; i++) {
-		state_node = of_parse_phandle(cpu_node, "cpu-idle-states", i);
+		state_node = of_get_cpu_state_node(cpu_node, i);
 		if (!state_node)
 			break;