ARM: mvebu: armada-xp-matrix: Relicense the device tree under GPLv2+/X11

The current GPL only licensing on the device tree 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 device trees under a GPL/X11 dual-license.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>