dt-bindings: pinctrl: Move mcp23s08 from gpio

The mcp23s08 driver was moved from gpio to pinctrl. This moves it's
devicetree binding doc as well. So driver and binding doc are in sync
again.
Signed-off-by: NLars Poeschel <poeschel@lemonage.de>
Reviewed-by: NSebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>