scsi: hisi_sas: Create root and device debugfs directories

This patch creates root directory at hisi_sas_init() and generates device
directory when we probe device driver.

And we remove the root directory at hisi_sas_exit(), but recursively
delete device directory when we remove device driver.

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>