Doing some digging it seems these SATA controllers are proprietary and don't support the standard AHCI/PCIIDE interfaces at all and requires a "native" driver.
You can try adding the following to OpenBSD's pciide.c and hoping some basic semblance of SATA support is to be found.. but it's highly unlikely.
Code:
const struct pciide_product_desc pciide_marvell_products[] = {
{ PCI_PRODUCT_MARVELL_88SX5081,
0,
sata_chip_map
},
};
FreeBSD indeed has a driver for this family of cards,
mvs(4), but it would appear you've run into other problems.