gpio: add GPIO support for SMSC SCH311x

This patch adds support for the GPIOs found on the SMSC "Super I/
SCH311x.

The chip detection and I/O functions are copied from sch311x_wdt.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>