gpio: bcm-kona: Implement get_direction callback

Implement gpio_chip's get_direction() callback, that lets other drivers get
particular GPIOs direction using gpiod_get_direction().

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>