bsps/clock: Use _SMP_Get_processor_maximum()

Use a specific test to enable the fast idle mode instead of using the
rtems_configuration_is_smp_enabled() workaround.

Update #3876.

bsps/shared/dev/clock/clockimpl.h

@@ -150,7 +150,7 @@ rtems_isr Clock_isr(
 
       Clock_driver_timecounter_tick();
 
-      if (!rtems_configuration_is_smp_enabled()) {
+      if (_SMP_Get_processor_maximum() == 1) {
         while (
           _Thread_Heir == _Thread_Executing && _Thread_Executing->is_idle
         ) {