gpio: ep93xx: Pass around struct gpio_chip

Instead of using a global variable, pass around the
struct gpio_chip * pointer and dereference to the state
container struct ep93xx_gpio as needed, like all other
drivers do.

Acked-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>