scsi: hisi_sas: Record the phy down event in debugfs

The number of phy down reflects the quality of the link between SAS
controller and disk. In order to allow the user to confirm the link quality
of the system, we record the number of phy down for each phy.

The user can check the current phy down count by reading the debugfs file
corresponding to the specific phy, or clear the phy down count by writing 0
to the debugfs file.

Link: https://lore.kernel.org/r/1571926105-74636-19-git-send-email-john.garry@huawei.com


Signed-off-by: Luo Jiaxing <luojiaxing@huawei.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>