LoongArch: Add Loongson-3 default config file

Add a default config file for LoongArch-based Loongson-3 platform.
Reviewed-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/Makefile

@@ -5,6 +5,8 @@
 
 boot	:= arch/loongarch/boot
 
+KBUILD_DEFCONFIG := loongson3_defconfig
+
 KBUILD_IMAGE	= $(boot)/vmlinux
 
 #