ARM: dts: imx6qdl-sabrelite: remove usdhc4 wp-gpio

On Sabre Lite usdhc4 is a micro sd slot, which has no
write protect.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>