dt-bindings: usb: snps,dwc3: Add missing 'dma-coherent' property

dt-bindings: usb: snps,dwc3: Add missing 'dma-coherent' property

The 'unevaluatedProperties' schema checks is not fully working and doesn't
catch some cases where there's a $ref to another schema. A fix is pending,
but results in new warnings in examples.

Some DWC3 implementations such as Xilinx are hooked up coherently and need
to set the 'dma-coherent' property.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220526014118.2872057-1-robh@kernel.org