scsi: hisi_sas: Add debugfs file structure for registers

Create a file structure which was used to save the memory address and
hisi_hba pointer for REGS at debugfs. This structure is bound to the
corresponding debugfs file, it can help callback function of debugfs file
to get what it need.

Link: https://lore.kernel.org/r/1571926105-74636-9-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>