[ARM] S3C: Fix gpio-config off-by-one bug

Fix gpio-config off-by-one bug. Without this patch, touching GPA0 pin on
S3C64XX platform causes kernel oops.

Reviewed-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>