gpio/samsung: only register available gpio banks

Only register gpio banks provided by SoC instead of the maximum possible
to lessen confusion, get rid of a warning from gpiolib and stop it from
eating into the extra gpios for configs with S3C24XX_GPIO_EXTRA != 0.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>