target/riscv: Move misa_mxl_max to class

target/riscv: Move misa_mxl_max to class

misa_mxl_max is common for all instances of a RISC-V CPU class so they
are better put into class.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240203-riscv-v11-2-a23f4848a628@daynix.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>