ahci: Enable SB600 64bit DMA on Asus M3A

Like e65cc194 this patch enables 64bit DMA
for the AHCI SATA controller of a board that has the SB600 southbridge. In
this case though we're enabling 64bit DMA for the Asus M3A motherboard. It
is a new enough board that all of the BIOS releases since the initial
release (0301 from 2007-10-22) work correctly with 64bit DMA enabled.

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