ramips: add SPDX license identifier into some dts,dtsi

Acked-by: Mathias Kresin <dev@kresin.me>
Acked-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Signed-off-by: NOGUCHI Hiroshi <drvlabo@gmail.com>

target/linux/ramips/dts/R6350.dts

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

target/linux/ramips/dts/WRC-1900GST.dts

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0
 /dts-v1/;
 
 #include "elecom_wrc-gst.dtsi"

target/linux/ramips/dts/elecom_wrc-gst.dtsi

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