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>