device: Add dev_of_node() accessor

Suggested by Arnd Bergmann, this gives a practical accessor for
the of_node field of struct device while instructing the compiler
that it will be NULL if CONFIG_OF is not set.
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/device.h

@@ -916,6 +916,13 @@ static inline void device_lock_assert(struct device *dev)
 	lockdep_assert_held(&dev->mutex);
 }
 
+static inline struct device_node *dev_of_node(struct device *dev)
+{
+	if (!IS_ENABLED(CONFIG_OF))
+		return NULL;
+	return dev->of_node;
+}
+
 void driver_init(void);
 
 /*