bindings: pinctrl-sx150x: Document SX1503 compatibility string

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/pinctrl/pinctrl-sx150x.txt

@@ -9,7 +9,8 @@ Required properties:
 			"semtech,sx1506q",
 			"semtech,sx1508q",
 			"semtech,sx1509q",
-			"semtech,sx1502q".
+			"semtech,sx1502q",
+			"semtech,sx1503q".
 
 - reg: The I2C slave address for this device.