gpio: mcp23s08: add support for mcp23018

This adds the required definitions 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>