ARM: SAMSUNG: Move GPIO common functions to plat-samsung

This patch moves GPIO common functions (from plat-s3c64xx) into plat-samsung.
and adds the config option to build the plat-samsung/gpiolib for Samsung SoCs.

Signed-off-by: Adityapratap Sharma <aditya.ps@samsung.com>
Signed-off-by: Atul Dahiya <atul.dahiya@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>