MIPS: Loongson64: fix spelling of SPDX tag

checkpatch looks for SPDX-License-Identifier.
So change the '_' to '-'

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/boot/dts/loongson/Makefile

-# SPDX_License_Identifier: GPL_2.0
+# SPDX-License-Identifier: GPL-2.0
 dtb-$(CONFIG_MACH_LOONGSON64)	+= loongson64_2core_2k1000.dtb
 dtb-$(CONFIG_MACH_LOONGSON64)	+= loongson64c_4core_ls7a.dtb
 dtb-$(CONFIG_MACH_LOONGSON64)	+= loongson64c_4core_rs780e.dtb