scsi: hisi_sas: Add BIST support for phy FFE

Add BIST support for phy FFE (Feed forward equalizer) setting. The user can
configure FFE through the new debugfs interface.

FFE is a parameter used for link layer control. It will affect the link
quality between the SAS controller and the backplane. In the BIST test, the
FFE interface is provided to assist board testers in optimizing link
parameters.

The modification of the FFE parameter will affect the test after BIST or
the normal running of the board. The user should save the initial FFE
values and restore them after BIST test is complete.

Link: https://lore.kernel.org/r/1598958790-232272-6-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>