riscv: qemu: Create a simple-bus driver for the soc node

riscv: qemu: Create a simple-bus driver for the soc node

To enumerate devices on the /soc/ node, create a "simple-bus"
driver to match "riscv-virtio-soc".

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Anup Patel <anup@brainfault.org>