scsi: stex: Add S6 support

1. Add reboot notifier and register it in stex_probe for all supported
   device.

2. For all supported device in restart flow, we get a callback from
   notifier and set S6flag for stex_shutdown & stex_hba_stop to send
   restart command to FW.

Signed-off-by: Charles.Chiou <charles.chiou@tw.promise.com>
Signed-off-by: Paul.Lyu <paul.lyu@tw.promise.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>