power: use C11 atomics for power state

power: use C11 atomics for power state

Since rte_atomicXX APIs are not allowed to be used, use C11 atomic
builtins for power in use state update.

Signed-off-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Acked-by: David Hunt <david.hunt@intel.com>