scsi_transport_sas: add 'scsi_target_id' sysfs attribute

There is no way to detect the scsi_target_id for any given SAS remote
port, so add a new sysfs attribute 'scsi_target_id'.

Signed-off-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Manoj Kumar <manoj@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>