scsi: Provide support for a list of AHCI controllers.

Many AHCI controllers are identical, the main (and often the
only) difference being the PCI Vendor ID/Device ID combination
reported by the device.

This change allows the config file to define a list of PCI vendor
ID/device ID pairs. The driver would scan the list and initialize
the first device it finds.

No actual multiple device list is introduced yet, this change
just add the framework.

Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Signed-off-by: Taylor Hutt <thutt@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>