doc: dt_qemu: correct dumpdtb description

Use only a single -machine parameter.

Describe that the same invocation of qemu-system-<arch> has to be
used for dumping the device-tree as will be used when executing U-Boot.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

doc/develop/devicetree/dt_qemu.rst

@@ -16,15 +16,22 @@ Obtaining the QEMU devicetree
 Where QEMU generates its own devicetree to pass to U-Boot you can use
 `-dtb u-boot.dtb` to force QEMU to use U-Boot's in-tree version.
 
-To obtain the devicetree that qemu generates, add `-machine dumpdtb=qemu.dtb`,
-e.g.::
+To obtain the devicetree that QEMU generates, add `dumpdtb=qemu.dtb` to the
+`-machine` argument, e.g.
 
-    qemu-system-arm -machine virt -machine dumpdtb=qemu.dtb
+.. code-block:: bash
 
-    qemu-system-aarch64 -machine virt -machine dumpdtb=qemu.dtb
-
-    qemu-system-riscv64 -machine virt -machine dumpdtb=qemu.dtb
+    qemu-system-aarch64 \
+      -machine virt,gic-version=3,dumpdtb=qemu.dtb \
+      -cpu cortex-a57 \
+      -smp 4 \
+      -memory 8G \
+      -chardev socket,id=chrtpm,path=/tmp/mytpm1/swtpm-sock \
+      -tpmdev emulator,id=tpm0,chardev=chrtpm \
+      -device tpm-tis-device,tpmdev=tpm0
 
+Except for the dumpdtb=qemu.dtb sub-parameter use the same qemu-system-<arch>
+invocation that you would use to start U-Boot to to get a complete device-tree.
 
 Merging in U-Boot nodes/properties
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