gpiolib: replace strict_strtol() with kstrtol()

The usage of strict_strtol() is not preferred, because
strict_strtol() is obsolete. Thus, kstrtol() should be
used.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>