Move the gpio.c code containing the core gpiolib and GPIO support to
plat-samsung from plat-s3c as it is used by all current Samsung SoCs.

Note, we didn't move this to gpiolib.c as it contains code that is not
strictly for gpiolib support and the 4bit code is already called gpiolib.c
so make the change easier by not renaming both files in one go.
Signed-off-by: NBen Dooks <ben-linux@fluff.org>