dm: ahci: Add a driver for SCSI on AHCI

dm: ahci: Add a driver for SCSI on AHCI

Some AHCI drivers use SCSI under the hood. Rather than making the AHCI
driver be in the SCSI uclass it makes sense to have the AHCI device create
a SCSI device as a child. That way we can handle any AHCI-specific
operations rather than trying to pretend tha the device is just SCSI.

To handle this we need to provide a way for AHCI drivers to bind a SCSI
device as its child, and probe it. Add functions for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>