[libata] ata_piix: Enable parallel scan

This patch turns on parallel scanning for the ata_piix driver.
This driver is used on most netbooks (no AHCI for cheap storage it seems).
The scan is the dominating time factor in the kernel boot for these
devices; with this flag it gets cut in half for the device I used
for testing (eeepc).
Alan took a look at the driver source and concluded that it ought to be safe
to do for this driver.  Alan has also checked with the hardware team.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>