Some ICH8s use non-linear port mapping.  ahci driver didn't use to
honor PORTS_IMPL and this made ports after hole nonfunctional.  This
patch makes ahci mark those ports as dummy and properly initialize all
the implemented ports after the dummies.

As it's unknown whether other AHCIs implement PORTS_IMPL register
properly, new board id board_ahci_pi is added and selectively applied
to ICH8s.  All other AHCIs continue to use linear mapping regardless
of PORTS_IMPL value.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Robin H. Johnson <robbat2@gentoo.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>