gpio: bcm-kona: 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: Markus Mayer <markus.mayer@linaro.org>
Cc: Tim Kryger <tim.kryger@linaro.org>
Cc: Matt Porter <matt.porter@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>