LoongArch: KVM: Add different length support in loongarch_pch_pic_write()

LoongArch: KVM: Add different length support in loongarch_pch_pic_write()

With function loongarch_pch_pic_write(), currently there is only four
bytes register write support. But in theory, all length 1/2/4/8 should
be supported for all the registers, here add different length support
about register write emulation in function loongarch_pch_pic_write().

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