ARM: dts: imx6: toradex: use SPDX-License-Identifier

1. Replace boiler plate licenses texts with the SPDX license
identifiers in Toradex iMX6-based SoM device trees.
2. As X11 is identical to the MIT License, but with an extra sentence
that prohibits using the copyright holders' names for advertising or
promotional purposes without written permission, use MIT license instead
of X11 ('s/X11/MIT/g').
3. Replace "Toradex AG" with "Toradex" in the Copyright notice.
4. Use GPL2.0+ instead of GPL2.0, as it's used now by default for all
new DTS files from Toradex.

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>