Introduce a PSCI DT helper function, psci_dt_attach_cpu(), which takes a
CPU number as an in-parameter and tries to attach the CPU's struct device
to its corresponding PM domain.

Let's makes use of dev_pm_domain_attach_by_name(), as it allows us to
specify "psci" as the "name" of the PM domain to attach to. Additionally,
let's also prepare the attached device to be power managed via runtime PM.

Signed-off-by: Ulf Hansson <redacted>
---
 drivers/cpuidle/cpuidle-psci-domain.c | 21 +++++++++++++++++++++
 drivers/cpuidle/cpuidle-psci.h        |  6 ++++++
 2 files changed, 27 insertions(+)

diff --git a/drivers/cpuidle/cpuidle-psci-domain.c b/drivers/cpuidle/cpuidle-psci-domain.c
index 3f5143ccc3e0..7429fd7626a1 100644
--- a/drivers/cpuidle/cpuidle-psci-domain.c
+++ b/drivers/cpuidle/cpuidle-psci-domain.c

@@ -9,9 +9,11 @@
 
 #define pr_fmt(fmt) "CPUidle PSCI: " fmt
 
+#include <linux/cpu.h>
 #include <linux/device.h>
 #include <linux/kernel.h>
 #include <linux/pm_domain.h>
+#include <linux/pm_runtime.h>
 #include <linux/psci.h>
 #include <linux/slab.h>
 #include <linux/string.h>

@@ -279,3 +281,22 @@ static int __init psci_idle_init_domains(void)
 	return ret;
 }
 subsys_initcall(psci_idle_init_domains);
+
+struct device *psci_dt_attach_cpu(int cpu)
+{
+	struct device *dev;
+
+	/* Currently limit the hierarchical topology to be used in OSI mode. */
+	if (!psci_has_osi_support())
+		return NULL;
+
+	dev = dev_pm_domain_attach_by_name(get_cpu_device(cpu), "psci");