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 <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Signed-off-by: Xiaofei Tan <[e-mail]>
Signed-off-by: John Garry <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

Documentation/devicetree/bindings/scsi/hisilicon-sas.txt

···
 Optional main node properties:
  - hip06-sas-v2-quirk-amt : when set, indicates that the v2 controller has the
 			    "am-max-transmissions" limitation.
+ - hisilicon,signal-attenuation : array of 3 32-bit values, containing de-emphasis,
+		preshoot, and boost attenuation readings for the board. They
+		are used to describe the signal attenuation of the board. These
+		values' range is 7600 to 12400, and used to represent -24dB to
+		24dB.
+		The formula is "y = (x-10000)/10000". For example, 10478
+		means 4.78dB.
 
 Example:
 	sas0: sas@c1000000 {