dt-bindings: usb: Convert Nuvoton EHCI to DT schema

The Nuvoton EHCI binding is just some compatible strings, so add it to the
generic-ehci.yaml schema.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230110-dt-usb-v3-5-5af0541fcf8c@kernel.org


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