LoongArch: Make CONFIG_64BIT as the default option

CONFIG_64BIT is the mandatory option before v7.0, but in v7.1-rc1 both
CONFIG_32BIT and CONFIG_64BIT are selectable and CONFIG_32BIT became the
default option. This breaks existing configurations, so explicitly make
CONFIG_64BIT as the default option to keep existing behavior.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

Author: chenhuacai

arch/loongarch/Kconfig

@@ -220,6 +220,7 @@ menu "Kernel type and options"
 
 choice
 	prompt "Kernel type"
+	default 64BIT # Keep existing behavior
 
 config 32BIT
 	bool "32-bit kernel"