ARM: dts: imx: add nitrogen6x board

Add file imx6q-nitrogen6x.dts,
	imx6dl-nitrogen6x.dts,
	imx6qdl-nitrogen6x.dtsi

And add board to makefile.

Eric Nelson created a web page to show the
differences between Nitrogen6x and Sabre Lite boards.
http://boundarydevices.com/differences-sabre-lite-nitrogen6x



Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>