i2c: i2c-cbus-gpio: Add vendor prefix to retu node in example

The example contains a device node for a retu device, but
its compatible string doesn't have a vendor prefix.

While being there, drop the -mfd suffix since isn't correct.

Signed-off-by: Javier Martinez Canillas <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Acked-by: Aaro Koskinen <[e-mail]>
Acked-by: Tony Lindgren <[e-mail]>
Reviewed-by: Wolfram Sang <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

Documentation/devicetree/bindings/i2c/i2c-cbus-gpio.txt

···
 	#address-cells = <1>;
 	#size-cells = <0>;
 
-	retu-mfd: retu@1 {
-		compatible = "retu-mfd";
+	retu: retu@1 {
+		compatible = "nokia,retu";
 		reg = <0x1>;
 	};
 };