ahci: Enable SB600 64bit DMA on MSI K9A2 Platinum v2

Like the Asus M2A-VM, MSI's K9A2 Platinum (MS-7376) can also support 64bit
DMA. It is a new enough board that all the BIOS releases work correctly with
64bit DMA enabled.

Signed-off-by: Mark Nelson <mdnelson8@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>