gpio: new driver to work with a 8x12 siox

This driver controls a SIOX device that provides 20 I/O lines. The first
twelve are fixed inputs, the remaining eight are outputs.

Acked-by: Gavin Schenk <g.schenk@eckelmann.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>