ARM: dts: imx6ul: prti6g: fix USB over-current detection on USB OTG port

USB over-current detection is supported on this board. So, activate it
and fixes the following kernel warnings:
 imx_usb 2184200.usb: No over current polarity defined
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6ul-prti6g.dts

@@ -177,6 +177,7 @@ &uart1 {
 
 &usbotg1 {
 	dr_mode = "host";
+	over-current-active-low;
 	status = "okay";
 };