I took a quick glance at the supported hardware for openbsd and did not see the Marvell SATA controllers listed. I recently tried to get a Via VT 8237S controller to support more than one SATA drive in OpenBSD without sucess.
I had better luck putting FreeBSD on the machine and it looks like FreeBSD added Marvell SATA support recently (8,1 release)
http://forums.freebsd.org/showthread.php?t=9533