ARM: dts: imx6ul-isiot: Add imx6ul-isiot-common.dtsi

lcdif nodes are differ wrt specific LCD connected on Is.IoT MX6UL
module, so create separate file 'imx6ul-isiot-common.dtsi' for common
lcdif node structure and include the same on respective dts.

More common nodes will add in future patches.

Cc: Matteo Lisi <matteo.lisi@engicam.com>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>