gpio: em: Setup gpiochip->dev

Make sure gpio_chip->dev is setup so of_gpiochip_add()
will work as expected.

Signed-off-by: Magnus Damm <damm@opensource.se>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>