dt-bindings: usb: dwc3: remove old DWC3 wrapper

There is now an updated bindings for these SoCs making the old
compatible obsolete.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>