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>

doc/board/emulation/qemu-riscv.rst

@@ -56,7 +56,7 @@ For instructions on how to run U-Boot in supervisor mode on QEMU
 with OpenSBI, see the documentation available with OpenSBI:
 https://github.com/riscv/opensbi/blob/master/docs/platform/qemu_virt.md
 
-These have been tested in QEMU 3.0.0.
+These have been tested in QEMU 4.2.0.
 
 Running U-Boot SPL
 ------------------