mediatek: add missing SPDX header

Signed-off-by: David Bauer <[email protected]>

target/linux/mediatek/dts/mt7981b-zyxel-nwa50ax-pro.dts

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: (GPL-2.0-or-later OR MIT)
 /dts-v1/;
 
 #include "mt7981.dtsi"