siox: add gpio bus driver

This bus driver uses GPIOs to control the four SIOX bus lines.

Acked-by: Gavin Schenk <g.schenk@eckelmann.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>