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.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>