ARM: dts: imx6ull: fix the imx6ull-14x14-evk configuration
imx6ull-14x14-evk.dts currently includes the imx6ul.dtsi file for an
i.MX6ULL SoC which is plain wrong.
Rename the current imx6ul-14x14-evk.dts to .dtsi and include it from
imx6ul-14x14-evk.dts and imx6ull-14x14-evk.dts, so that both can
include the appropriate SoC specific (imx6ul.dtsi/imx6ull.dtsi) file.