pata_atiixp: Don't use unconnected secondary port on SB600/SB700

The SB600 and SB700 southbridge chips from ATI/AMD only have
connections for the primary IDE port. As these chips have unique
pci device ID's use these to mark the secondary port as 'dummy'

Signed-off-by: Darren Stevens <darren@stevens-zone.net>
Signed-off-by: Tejun Heo <tj@kernel.org>