scsi: hisi_sas: Debugfs global register create file and add file operations

This patch create debugfs file for global register and add file
operations.

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>