gpio: siox: Use devm_ managed gpiochip

By using devm_gpiochip_add_data() we can get rid of the
remove() callback. As this driver doesn't use the
gpiochip data pointer we simply pass in NULL.

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