gpio: mcp23s08: move to pinctrl

This moves the mcp23s08 driver from gpio to pinctrl. Actual
pinctrl support for configuration of the pull-up resistors
follows in its own patch.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Acked-by: Sylvain Lemieux <slemieux.tyco@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>