LoongArch: KVM: Use generic function loongarch_eiointc_read()

Generic read function loongarch_eiointc_read() is used for 1/2/4/8
bytes read access. It reads 8 bytes from emulated software state and
shift right from address offset.

Also the similar with kvm_complete_iocsr_read(), destination register
of IOCSRRD.{B/H/W} is sign extension from byte/half word/word.

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