docs: aspeed: Give an example of booting a kernel

A common use case for the ASPEED machine is to boot a Linux kernel.
Provide a full example command line.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Message-id: 20211117065752.330632-4-joel@jms.id.au
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

docs/system/arm/aspeed.rst

@@ -78,9 +78,9 @@ Missing devices
 Boot options
 ------------
 
-The Aspeed machines can be started using the ``-kernel`` option to
-load a Linux kernel or from a firmware. Images can be downloaded from
-the OpenBMC jenkins :
+The Aspeed machines can be started using the ``-kernel`` and ``-dtb`` options
+to load a Linux kernel or from a firmware. Images can be downloaded from the
+OpenBMC jenkins :
 
    https://jenkins.openbmc.org/job/ci-openbmc/lastSuccessfulBuild/
 
@@ -88,6 +88,15 @@ or directly from the OpenBMC GitHub release repository :
 
    https://github.com/openbmc/openbmc/releases
 
+To boot a kernel directly from a Linux build tree:
+
+.. code-block:: bash
+
+  $ qemu-system-arm -M ast2600-evb -nographic \
+        -kernel arch/arm/boot/zImage \
+        -dtb arch/arm/boot/dts/aspeed-ast2600-evb.dtb \
+        -initrd rootfs.cpio
+
 The image should be attached as an MTD drive. Run :
 
 .. code-block:: bash