hw/riscv: virt: Add optional ACLINT support to virt machine

hw/riscv: virt: Add optional ACLINT support to virt machine

We extend virt machine to emulate ACLINT devices only when "aclint=on"
parameter is passed along with machine name in QEMU command-line.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 20210831110603.338681-5-anup.patel@wdc.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>