scsi: cxlflash: Staging to support future accelerators

As staging to support future accelerator transports, add a shim layer
such that the underlying services the cxlflash driver requires can be
conditional upon the accelerator infrastructure.

Signed-off-by: Matthew R. Ochs <mrochs@linux.vnet.ibm.com>
Signed-off-by: Uma Krishnan <ukrishn@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>