gpio: Do not include <linux/kernel.h> when not really needed.

<linux/kernel.h> is included only for using container_of().
Include <linux/container_of.h> instead, it is much lighter.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>