ARM: dts: imx6-tx6: Relicense the Ka-Ro DT files under GPLv2/X11

GPLv2-only devicetrees make reuse difficult for software components
licensed under a different license.

The consensus is that a GPL/X11 dual-license should allow all necessary
uses, so relicense the imx6*-tx6* files to this combination.

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