doc: board: qemu-ppce500: Document eTSEC usage

Document how to launch a QEMU session with eTSEC as a network device.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>
diff --git a/doc/board/emulation/qemu-ppce500.rst b/doc/board/emulation/qemu-ppce500.rst
index 0a5c86c..5de0aaf 100644
--- a/doc/board/emulation/qemu-ppce500.rst
+++ b/doc/board/emulation/qemu-ppce500.rst
@@ -70,6 +70,11 @@
     $ qemu-system-ppc -nographic -machine ppce500 -bios u-boot \
                       -nic tap,ifname=tap0,script=no,downscript=no,model=e1000
 
+The QEMU ppce500 machine can also dynamically instantiate an eTSEC device if
+"-device eTSEC" is given to QEMU::
+
+    -netdev tap,ifname=tap0,script=no,downscript=no,id=net0 -device eTSEC,netdev=net0
+
 VirtIO BLK driver is also enabled to support booting from a disk image where
 a kernel image is stored. Append the following to QEMU::