superio/ite/it8718f: Remove hard coding from romstage

Make use of the ITE common Super I/O framework and there-by removing any
hard coding of Super I/O base address.

Change-Id: I14af89d2727d7c6bac0f9840043c430726297429
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/5717
Tested-by: build bot (Jenkins)
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>