kernel/reboot: Add stub for pm_power_off

Add weak stub for the global pm_power_off callback variable. This will
allow us to remove pm_power_off definitions from arch/ code and transition
to the new sys-off based API that will replace the global variable.
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

kernel/reboot.c

@@ -57,6 +57,12 @@ struct sys_off_handler {
 	void *list;
 };
 
+/*
+ * Temporary stub that prevents linkage failure while we're in process
+ * of removing all uses of legacy pm_power_off() around the kernel.
+ */
+void __weak (*pm_power_off)(void);
+
 /*
  * If set, this is used for preparing the system to power off.
  */