scsi: cxlflash: Support process specific mappings

Once the context is started, the assigned MMIO space can be mapped and
unmapped. Provide means to map and unmap the context MMIO space.

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