regulator: Fix regulator_register() API signature in Documentation

The commit 2c043bcb ("regulator: pass additional of_node to
regulator_register()") added an additional parameter to the
regulator_register() API.
Update the Documentation accordingly to reflect the change
in the function signature.
Reported-by: Thomas Abraham <thomas.abraham@linaro.org>
Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

Documentation/power/regulator/regulator.txt

@@ -12,7 +12,7 @@ Drivers can register a regulator by calling :-
 
 struct regulator_dev *regulator_register(struct regulator_desc *regulator_desc,
 	struct device *dev, struct regulator_init_data *init_data,
-	void *driver_data);
+	void *driver_data, struct device_node *of_node);
 
 This will register the regulators capabilities and operations to the regulator
 core.