gpio: samsung: add devicetree init for s3c24xx arches

Until now the EXYNOS-SoC was the only Samsung-SoC supporting the GPIOs
via the device tree. This patch implements dt-support for the
s3c24xx arches.

The controllers contain only 3 cells, as the underlying gpio controller
does not support controlling the drive strength on a gpio level.

Tested with the gpio-keys driver on a s3c2416 based machine.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Thomas Abraham <thomas.abraham@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
[kgene.kim@samsung.com: fixed build error]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>