dt-bindings: usb: dwc3: add description for rk3328

Add description for "rockchip,rk3328-dwc3".

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210209192350.7130-6-jbx6244@gmail.com


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