ramips: add missing SPDX identifier for EX6150

This adds the SPDX license identifier for the NETGEAR EX6150. It was
missed when submitting the original patch.

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

target/linux/ramips/dts/EX6150.dts

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0-or-later OR MIT
 /dts-v1/;
 
 #include "mt7621.dtsi"