pinctrl: samsung/s3c24xx: add CONFIG_OF dependency

The driver fails to build without CONFIG_OF:

drivers/pinctrl/samsung/pinctrl-samsung.c: In function 'samsung_gpiolib_register':
drivers/pinctrl/samsung/pinctrl-samsung.c:936:5: error: 'struct gpio_chip' has no member named 'of_node'

This configuration is now possible since we can now select the
PINCTRL subsystem on S3C24xx machines other than the device tree
based ones.

Fixes: d219b924 ("pinctrl: change Kconfig PINCTRL variable to a menuconfig")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>