forked from Mirrors/opensbi

Since QEMU commit 8b1d0714bfdd "riscv: sifive_u: Allow up to 4 CPUs to be created", the maximum number of hart for sifive_u is 4. Change our hart count to match that. Signed-off-by: Bin Meng <bmeng.cn@gmail.com> Reviewed-by: Atish Patra <atish.patra@wdc.com>