arm64: dts: imx8mq-librem5: remove undocument property 'extcon' for usb-pd@3f

arm64: dts: imx8mq-librem5: remove undocument property 'extcon' for usb-pd@3f

Remove undocment property 'extcon' for usb-pd@3f to fix below CHECK_DTBS
warnings:
arch/arm64/boot/dts/freescale/imx8mq-librem5-r2.dtb: usb-pd@3f: 'extcon' does not match any of the regexes: 'pinctrl-[0-9]+'
        from schema $id: http://devicetree.org/schemas/usb/ti,tps6598x.yaml#

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>