dt-bindings: usb: add documentation for aspeed udc

Add device tree binding documentation for the Aspeed USB2.0 Device
Controller.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Neal Liu <neal_liu@aspeedtech.com>
Link: https://lore.kernel.org/r/20220523030134.2977116-4-neal_liu@aspeedtech.com


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