hw/riscv/spike: Allow more than one CPUs

hw/riscv/spike: Allow more than one CPUs

Currently, the upstream Spike ISA simulator allows more than
one CPUs so we update QEMU Spike machine on similar lines to
allow more than one CPUs.

The maximum number of CPUs for QEMU Spike machine is kept
same as QEMU Virt machine.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20200427080644.168461-4-anup.patel@wdc.com
Message-Id: <20200427080644.168461-4-anup.patel@wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>