With heterogeneous harts config, the PLIC hart topology configuration
string are "M,MS,.." because of the monitor hart #0.
Suggested-by: Fabien Chouteau <chouteau@adacore.com>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>