scsi: hisi_sas: dt-bindings: add an property of signal attenuation

For some new boards with hip07 chipset we are required to set PHY config
registers differently. The hw property which determines how to set these
registers is in the PHY signal attenuation readings.

This patch add an devicetree property, "hisilicon,signal-attenuation", which
is used to describe the signal attenuation of an board.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Xiaofei Tan <tanxiaofei@huawei.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>