ARM: dts: imx6ul-kontron-n6x1x-s: Add vbus-supply and overcurrent polarity to usb nodes

To silence the warnings shown by the driver at boot time, we add a
fixed regulator for the 5V supply of usbotg2 and specify the polarity
of the overcurrent signal for usbotg1.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>