scsi: hisi_sas: support the property of signal attenuation for v2 hw

The register SAS_PHY_CTRL is configured according to signal quality.  The
signal quality is calculated by signal attenuation of hardware physical
link. It may be different for different PCB layout.

So, in order to give better support to new board, this patch add support to
reading the devicetree property, "hisilicon,signal-attenuation".  Of course,
we still keep an default value in driver to adapt old board.

Signed-off-by: Xiaofei Tan <tanxiaofei@huawei.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>