ARM: dts: rename imx6q.dtsi to imx6qdl.dtsi

i.MX6 Quad and i.MX6 DualLite is similar enough to share one dtsi
file, so rename imx6q.dtsi to imx6qdl.dtsi preparing for the addition
of imx6dl support.

Another member of i.MX6 series i.MX6 SoloLite is different enough
from the other two, so it will stand as a separate dtsi.  That's why
we rename to imx6qdl.dtsi not imx6.dtsi.

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