ARM: dts: imx: make sabreauto and sabresd common

The sabreauto and sabresd boards are common for imx6q and imx6dl.
Create imx6qdl-sabreauto.dtsi and imx6qdl-sabresd.dtsi for those
common parts.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>