dt-bindings: usb: dwc3: Add Tx De-emphasis constraints

In accordance with the driver comments the PIPE3 de-emphasis can be tuned
to be either -6dB, -2.5dB or disabled. Let's add the de-emphasis
property constraints so the DT schema would make sure the controller DT
node is equipped with correct value.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Link: https://lore.kernel.org/r/20201210090944.16283-14-Sergey.Semin@baikalelectronics.ruSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/snps,dwc3.yaml

@@ -156,6 +156,10 @@ properties:
       The value driven to the PHY is controlled by the LTSSM during USB3
       Compliance mode.
     $ref: /schemas/types.yaml#/definitions/uint8
+    enum:
+      - 0 # -6dB de-emphasis
+      - 1 # -3.5dB de-emphasis
+      - 2 # No de-emphasis
 
   snps,dis_u3_susphy_quirk:
     description: When set core will disable USB3 suspend phy