LoongArch: KVM: Add vcpu related header files

Add LoongArch vcpu related header files, including vcpu csr information,
irq number definitions, and some vcpu interfaces.

Reviewed-by: Bibo Mao <maobibo@loongson.cn>
Tested-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Tianrui Zhao <zhaotianrui@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>