LoongArch: KVM: Add PCHPIC user mode read and write functions

LoongArch: KVM: Add PCHPIC user mode read and write functions

Implement the communication interface between the user mode programs
and the kernel in PCHPIC interrupt control simulation, which is used
to obtain or send the simulation data of the interrupt controller in
the user mode process, and is also used in VM migration or VM saving
and restoration.

Signed-off-by: Tianrui Zhao <zhaotianrui@loongson.cn>
Signed-off-by: Xianglai Li <lixianglai@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>