dt-bindings: pinctrl: mcp23s08: add documentation for drive-open-drain

This flag set the mcp23s08 device irq type to open drain active low.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>