target/riscv: Validate the mode in write_vstvec

target/riscv: Validate the mode in write_vstvec

Base on the riscv-privileged spec, vstvec substitutes for the usual stvec.
Therefore, the encoding of the MODE should also be restricted to 0 and 1.

Signed-off-by: Jiayi Li <lijiayi@eswincomputing.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Message-ID: <20240701022553.1982-1-lijiayi@eswincomputing.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>