riscv: qemu: Remove the simple-bus driver for the SoC node

Prior to QEMU v3.1.0, QEMU generated the 'virt' SoC node with a
"riscv-virtio-soc" compatible string, and a "simple-bus" driver
was created to accommodate that special case in U-Boot.

Starting from QEMU v3.1.0, the SoC node was set as a "simple-bus",
hence the special simple-bus driver is no longer needed.

Update the doc to mention the latest tested QEMU version 4.2.0.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>