northbridge: DEVICE_NOOP some stub function callbacks

Reduces loc and makes NOP's explicit.

Change-Id: I8a117b150b8b421c7a18b48a2ac36d15679f20b0
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7344
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>