[ARM] S3C64XX: Fix missing definition of s3c64xx_init_io()

The function s3c64xx_init_io was missing from <plat/cpu.h>
and was masked by the SMDK6410 having an local definition.

Fix by removing the SMDK6410 variant and adding it to the
relevant <plat/cpu.h> file.

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