of: add helper function to retrive match data

It's a common operation for device drivers to retrive the data
member from of_device_id struct in their probe function.

Most driver end up doing:
const struct of_device_id *match;
match = of_match_device(driver_of_match, &pdev->dev);
driver->data = match->data;

With the of_device_get_match_data helper function all this can
done in one go.

Signed-off-by: Joachim Eastwood <[e-mail]>
[robh: add missing inline to dummmy declaration]
Signed-off-by: Rob Herring <[e-mail]>

drivers/of/device.c

···
 }
 EXPORT_SYMBOL(of_device_unregister);
 
+const void *of_device_get_match_data(const struct device *dev)
+{
+	const struct of_device_id *match;
+
+	match = of_match_device(dev->driver->of_match_table, dev);
+	if (!match)
+		return NULL;
+
+	return match->data;
+}
+EXPORT_SYMBOL(of_device_get_match_data);
+
 ssize_t of_device_get_modalias(struct device *dev, char *str, ssize_t len)
 {
 	const char *compat;

include/linux/of_device.h

···
 extern int of_device_register(struct platform_device *ofdev);
 extern void of_device_unregister(struct platform_device *ofdev);
 
+extern const void *of_device_get_match_data(const struct device *dev);
+
 extern ssize_t of_device_get_modalias(struct device *dev,
 					char *str, ssize_t len);
 
···
 
 static inline void of_device_uevent(struct device *dev,
 			struct kobj_uevent_env *env) { }
+
+static inline const void *of_device_get_match_data(const struct device *dev)
+{
+	return NULL;
+}
 
 static inline int of_device_get_modalias(struct device *dev,
 				   char *str, ssize_t len)