dts: imx8mq-librem5: usb_dwc3_1: Limit to USB 2.0
Not sure if it's needed or wanted, whether it has power consumption implications or whether it may help #303 (closed), but since we're only using USB 2.0 on the internal bus maybe it's good to explicitly disable USB 3.0 there? (kinda inspired by !377 (merged))