target/loongarch: Set unused flag with CSR registers

target/loongarch: Set unused flag with CSR registers

On LA464, some CSR registers are not used such as CSR_SAVE8 -
CSR_SAVE15, also CSR registers relative with MCE is not used now.

Flag CSRFL_UNUSED is added for these registers, so that it will
not dumped. In order to keep compatiblity, these CSR registers are
not removed since it is used in vmstate already.

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