[ARM] S3C24XX: GPIO: Move gpiolib initialisation earlier

The arch_initcall() is too late for board initialisation to use
gpiolib when doing their machine specific initilisation via the
.init_machine callback.

Bring the file into line with the s3c64xx implementation and use
the core_initcall() to register the GPIO chips.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>