ath79: ubnt-bullet-m-xw: Change DTS license to GPL-2.0-or-later OR MIT

Having MIT as alternative is sometimes preferred by upstream maintainers
and allows sharing that simple code with other projects. We don't really
want multiple DTS versions for the same device.

Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/ath79/dts/ar9342_ubnt_bullet-m-xw.dts

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: GPL-2.0-or-later OR MIT
 /dts-v1/;
 
 #include <dt-bindings/gpio/gpio.h>