ARM: dts: imx28-tx28: Relicense the TX28 dts file under GPLv2/X11

The current GPL only licensing on the dts file makes it very
impractical for other software components licensed under another
license.

In order to make it easier for them to reuse our device trees,
relicense our dts files first under a GPL/X11 dual-license.

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