treewide: Add SPDX license identifier - Kbuild

Add SPDX license identifiers to all Make/Kconfig files which:

 - Have no license information of any form

These files fall under the project license, GPL v2 only. The resulting SPDX
license identifier is:

      GPL-2.0

Reported-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Kate Stewart <kstewart@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/mips/include/asm/Kbuild

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0
 # MIPS headers
 generated-y += syscall_table_32_o32.h
 generated-y += syscall_table_64_n32.h

arch/mips/include/uapi/asm/Kbuild

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0
 generated-y += unistd_n32.h
 generated-y += unistd_n64.h
 generated-y += unistd_o32.h