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: NSimon Glass <sjg@chromium.org>
Reviewed-by: NBin Meng <bmeng.cn@gmail.com>