ARM: S3C24XX: Remove obsolete GPIO API declarations

It's safe to empty the plat-samsung/incude/plat/gpio-fns.h header
now when all users of those functions have been converted to
generic gpiolib API or to s3c_gpio_cfg().

Cc: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Sylwester Nawrocki <sylvester.nawrocki@gmail.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>