pinctrl: cygnus-gpio: use gpiochip data pointer

This makes the driver use the data pointer added to the gpio_chip
to store a pointer to the state container instead of relying on
container_of().

Cc: bcm-kernel-feedback-list@broadcom.com
Cc: Scott Branden <sbranden@broadcom.com>
Cc: Jon Mason <jonmason@broadcom.com>
Acked-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>