cpuidle: psci: Enable the hierarchical topology for s2idle on PREEMPT_RT

To enable the domain-idle-states to be used during s2idle on a PREEMPT_RT
based configuration, let's allow the re-assignment of the ->enter_s2idle()
callback to psci_enter_s2idle_domain_idle_state().

Similar to s2ram, let's leave the support for CPU hotplug outside
PREEMPT_RT, as it's depending on using runtime PM. For s2idle, this means
that an offline CPU's PM domain will remain powered-on. In practise this
may lead to that a shallower idle-state than necessary gets selected, which
shouldn't be an issue (besides wasting power).
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Raghavendra Kakarla <quic_rkakarla@quicinc.com>  # qcm6490 with PREEMPT_RT set
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/20240527142557.321610-8-ulf.hansson@linaro.org

drivers/cpuidle/cpuidle-psci.c

@@ -233,18 +233,17 @@ static int psci_dt_cpu_init_topology(struct cpuidle_driver *drv,
 
 	psci_cpuidle_use_syscore = true;
 
-	/* The hierarchical topology is limited to s2ram on PREEMPT_RT. */
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		return 0;
-
 	/*
 	 * Using the deepest state for the CPU to trigger a potential selection
 	 * of a shared state for the domain, assumes the domain states are all
-	 * deeper states.
+	 * deeper states. On PREEMPT_RT the hierarchical topology is limited to
+	 * s2ram and s2idle.
 	 */
-	drv->states[state_count - 1].enter = psci_enter_domain_idle_state;
 	drv->states[state_count - 1].enter_s2idle = psci_enter_s2idle_domain_idle_state;
-	psci_cpuidle_use_cpuhp = true;
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
+		drv->states[state_count - 1].enter = psci_enter_domain_idle_state;
+		psci_cpuidle_use_cpuhp = true;
+	}
 
 	return 0;
 }