riscv: sifive_u: Set the minimum number of cpus to 2

riscv: sifive_u: Set the minimum number of cpus to 2

It is not useful if we only have one management CPU.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
[Palmer: Set default CPUs to 2]
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>