cpuidle / ux500 : use CPUIDLE_FLAG_TIMER_STOP flag

Use the CPUIDLE_FLAG_TIMER_STOP and let the cpuidle framework
to handle the CLOCK_EVT_NOTIFY_BROADCAST_ENTER/EXIT when entering
this state.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>