target/riscv/cpu.c: add riscv_cpu_validate_misa_mxl()

target/riscv/cpu.c: add riscv_cpu_validate_misa_mxl()

Let's remove more code that is open coded in riscv_cpu_realize() and put
it into a helper. Let's also add an error message instead of just
asserting out if env->misa_mxl_max != env->misa_mlx.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230517135714.211809-9-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>