pinctrl: mcp23s08: fix comment for mcp23s08_platform_data.base

The comment does not match the driver, which actually supports
automatic assignment. Fix this by updating the comment.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>