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>