ARM: dts: imx6-tx6: move display configuration to .dtsi file

Currently the display (LCD or LVDS) configuration is duplicated with
slight variations in each TX6 module specific file.

Move it to an include file for simplification and consistency.

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