TPM: Fix i2c driver dependency

Change-Id: I59545ef734dff41ba55dcddd541c54b17b0855bb
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/7914
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>