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

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.ru
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>