pinctrl: mcp23s08: add pinconf support

mcp23xxx device have configurable 100k pullup resistors. This adds
support for enabling them using pinctrl's pinconf interface.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>