ARM: dts: imx6dl: alti6p: fix different USB related warnings

Fix USB-related warnings in imx6dl alti6p device tree by disabling
unused usbphynop1 and usbphynop2 USB PHYs and properly configuring
over-current detection on related ports. This fixes the following
warnings with the current kernel:
usb_phy_generic usbphynop1: dummy supplies not allowed for exclusive requests
usb_phy_generic usbphynop2: dummy supplies not allowed for exclusive requests
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>