dt-bindings: pinctrl: mcp23s08 update binding doc

The mcp23s08 driver moved to pinctrl recently. It accepts the
bias-pull-up pinctrl property since then. This updates the binding
doc to reflect that.

Thanks to Sebastian Reichel for the working example.

Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>