scsi: hisi_sas: Add debugfs file structure for IOST cache

scsi: hisi_sas: Add debugfs file structure for IOST cache

Create a file structure which was used to save the memory address for IOST
cache at debugfs. This structure is bound to the corresponding debugfs
file, it can help callback function of debugfs file to get what it needs.

Link: https://lore.kernel.org/r/1571926105-74636-13-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>