dt-bindings: pinctrl: add mcp23018 to mcp23s08 documentation

This adds the compatible string for the mcp23018, which is the i2c variant
of the mcp23s18.
Signed-off-by: Phil Reid <preid@electromag.com.au>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/pinctrl/pinctrl-mcp23s08.txt

@@ -13,6 +13,7 @@ Required properties:
     - "microchip,mcp23s18" for 16 GPIO SPI version
     - "microchip,mcp23008" for  8 GPIO I2C version or
     - "microchip,mcp23017" for 16 GPIO I2C version of the chip
+    - "microchip,mcp23018" for 16 GPIO I2C version
     NOTE: Do not use the old mcp prefix any more. It is deprecated and will be
     removed.
 - #gpio-cells : Should be two.