pm: Implement function stubs when PM is not enabled

Create stubs for when the subsystem is not compiled. Some drivers need
to use constraints when are in the middle of a transaction to avoid
the system to sleep. As the same driver may be used in a target that
does not support power management, they have to conditionally (#if
CONFIG_PM) call these functions, these stubs avoid this regular problem.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>

include/pm/pm.h

diff --git a/include/pm/pm.h b/include/pm/pm.h
index 2ce2b7b..184bc11 100644
--- a/include/pm/pm.h
+++ b/include/pm/pm.h
@@ -23,8 +23,6 @@
  * @}
  */
 
-#ifdef CONFIG_PM
-
 /**
  * @brief System Power Management API
  *
@@ -65,6 +63,7 @@
 	void (*state_exit)(enum pm_state state);
 };
 
+#ifdef CONFIG_PM
 /**
  * @brief Force usage of given power state.
  *
@@ -202,6 +201,19 @@
 
 
 void z_pm_save_idle_exit(int32_t ticks);
+
+#else  /* CONFIG_PM */
+
+#define pm_notifier_register(notifier)
+#define pm_notifier_unregister(notifier) (-ENOSYS)
+
+#define pm_constraint_set(pm_state)
+#define pm_constraint_release(pm_state)
+#define pm_constraint_get(pm_state) (true)
+
+#define pm_power_state_set(info)
+#define pm_power_state_exit_post_ops(info)
+
 #endif /* CONFIG_PM */
 
 #ifdef __cplusplus