ARM: s3c64xx: Use the right include

The file s3c64xx.c is including <linux/gpio.h> despite using no
symbols from the file, however it needs it to implicitly bring in
of_have_populated_dt() so include <linux/of.h> explicitly instead.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>