platform: qemu/sifive_u: Update the hart count to 4

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>