LoongArch: Set CPU#0 as the io master for FDT

ACPI systems set io masters by parsing ACPI MADT, FDT systems have no
MADT so we explicitly set CPU#0 as the io master. Otherwise CPU#0 will
be considered as hotpluggable.

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