sata_mv HighPoint 2310 support (88SX7042)

With the following patch, my HighPoint 2310 with a Marvell 88SX7042 on
it seems to work OK.

The controller only has 4 ports, with MV_FLAG_DUAL_HC it seems to init 8
ports and fails miserably at probe time. There are no other devices mapped
to that chip, maybe it was just incorrectly specified in the first place?

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>