Add basic port-multiplier support to sata_mv.
This works in Command-based-switching mode for Gen-II chipsets,
and in FIS-based-switching mode for Gen-IIe chipsets.

Error handling remains at the primary port level for now
(works okay, but not great).  This will get fixed in a subsequent
patch series for IRQ/EH handling fixes.  There are also some
known NCQ/PMP errata to be dealt with in the near future,
once we have this basic PMP support in place.
Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>