ARM: dts: imx28-tx28: add SPDX-License-Identifier

Replace the license blob by a clean SPDX-License-Identifier with GPL2
or MIT even if X11 is specified in the original blob since the actual
license text corresponds to a MIT license.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Cc: Lothar Waßmann <LW@KARO-electronics.de>
Acked-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>