MIPS:DTS:Correct the license for Loongson-2K

make license from GPL3.0 to GPL2.0
Signed-off-by: xiaochuan mao <maoxiaochuan@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/boot/dts/loongson/loongson64-2k1000.dtsi

-// SPDX-License-Identifier: GPL-3.0
+// SPDX-License-Identifier: GPL-2.0
 
 /dts-v1/;