PM / Sleep: introduce dpm_for_each_dev

dpm_list and its pm lock provide a good way to iterate all
devices in system. Except this way, there is no other easy
way to iterate devices in system.

firmware loader need to cache firmware images for devices
before system sleep, so introduce the function to meet its
demand.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/include/linux/pm.h b/include/linux/pm.h
index f067e60..88f034a 100644
--- a/include/linux/pm.h
+++ b/include/linux/pm.h
@@ -638,6 +638,7 @@
 	} while (0)
 
 extern int device_pm_wait_for_dev(struct device *sub, struct device *dev);
+extern void dpm_for_each_dev(void *data, void (*fn)(struct device *, void *));
 
 extern int pm_generic_prepare(struct device *dev);
 extern int pm_generic_suspend_late(struct device *dev);
@@ -677,6 +678,10 @@
 	return 0;
 }
 
+static inline void dpm_for_each_dev(void *data, void (*fn)(struct device *, void *))
+{
+}
+
 #define pm_generic_prepare	NULL
 #define pm_generic_suspend	NULL
 #define pm_generic_resume	NULL