pinctrl: mcp23s08: drop pullup config from pdata

mcp23s08 support configuration of the pullups using the
pinconf framework. This removes the custom pullup configuration
from platform data, which has no upstream users.

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