dt-bindings: usb: Add Gen2 Tx Compliance tuning property for dwc3

Tuning for Tx 3.1 Gen2 needs to be done by modifiying the deemph
registers. Expose these registers via device tree properties so that
tuning can be applied duing dwc3 init.

Change-Id: I84066e976ccbd6006ca9149e29d1c27acb694646