hw/riscv/virtio: Set the soc device tree node as a simple-bus

hw/riscv/virtio: Set the soc device tree node as a simple-bus

To allow Linux to enumerate devices on the /soc/ node set it as a
"simple-bus".

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>