dt-bindings: usb: do not use deprecated synopsys prefix

The "synopsys" prefix and "synopsys,dwc3" compatible are deprecated.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220221082228.34407-3-krzysztof.kozlowski@canonical.com