The GW7901 has USB2 routed to a USB VBUS supply with over-current
protection via an active-low pin. Define the OC pin polarity properly.

Fixes: 2b1649a8 ("arm64: dts: imx: Add i.mx8mm Gateworks gw7901 dts support")
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>