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

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

With function loongarch_pch_pic_read(), currently it is hardcoded length
for different registers, and the length comes from exising linux pch_pic
driver code. But in theory, all length 1/2/4/8 should be supported for
all the registers, here add different length support about register read
emulation in function loongarch_pch_pic_read().

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