staging: mt7621-dts: add missing SPDX license to files

Device tree files included here haven't got SPDX license header at all.
Hence, add it to all of them.
Signed-off-by: NSergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20211025094240.4356-1-sergio.paracuellos@gmail.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/mt7621-dts/gbpc1.dts

+// SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
 /dts-v1/;
 
 #include "mt7621.dtsi"

drivers/staging/mt7621-dts/gbpc2.dts

+// SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
 /dts-v1/;
 
 #include "gbpc1.dts"

drivers/staging/mt7621-dts/mt7621.dtsi

+// SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
 #include <dt-bindings/interrupt-controller/mips-gic.h>
 #include <dt-bindings/gpio/gpio.h>
 #include <dt-bindings/clock/mt7621-clk.h>