cpuidle: psci: 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().

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>