ARM: dts: imx6qdl-sabreauto: Support debug LED

The D9 LED controlled by gpio on the imx6qdl-sabreauto
CPU board is a debug LED according to the board design.
This patch adds the relevant device tree nodes to the
imx6qdl-sabreauto device tree file to support this LED.

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>