imx: imx8m: soc: runtime drop extcon property from usbotg node

The extcon is an decrepted property and not used by upstream Linux and
NXP 5.10 kernel, so we remove it before kicking linux in case it is in
dts. Otherwise distro kernel will not able to have usb function.

Reviewed-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>