pm: move private enum to source using it

`enum device_pm_state` is not used publicly, so move it to the source
using it.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
diff --git a/include/pm/device_runtime.h b/include/pm/device_runtime.h
index df919e9..c2baf72 100644
--- a/include/pm/device_runtime.h
+++ b/include/pm/device_runtime.h
@@ -25,14 +25,6 @@
 
 #ifdef CONFIG_PM_DEVICE_IDLE
 
-/* Device PM states */
-enum device_pm_state {
-	DEVICE_PM_STATE_ACTIVE = 1,
-	DEVICE_PM_STATE_SUSPENDED,
-	DEVICE_PM_STATE_SUSPENDING,
-	DEVICE_PM_STATE_RESUMING,
-};
-
 /**
  * @brief Enable device idle PM
  *
diff --git a/subsys/pm/device_pm.c b/subsys/pm/device_pm.c
index 4bd382d..65daeee 100644
--- a/subsys/pm/device_pm.c
+++ b/subsys/pm/device_pm.c
@@ -14,6 +14,14 @@
 #include <logging/log.h>
 LOG_MODULE_DECLARE(power);
 
+/* Device PM states */
+enum device_pm_state {
+	DEVICE_PM_STATE_ACTIVE = 1,
+	DEVICE_PM_STATE_SUSPENDED,
+	DEVICE_PM_STATE_SUSPENDING,
+	DEVICE_PM_STATE_RESUMING,
+};
+
 /* Device PM request type */
 #define DEVICE_PM_SYNC			(0 << 0)
 #define DEVICE_PM_ASYNC			(1 << 0)