gpio: samsung: Use CONFIG_ARCH_S3C64XX to check for S3C64XX support

Since CONFIG_PLAT_S3C64XX is going to be removed, this patch modifies
the gpio-samsung driver to use the proper way of checking for S3C64xx
support - CONFIG_ARCH_S3C64XX.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>