doc: qemu: debug UART settings for QEMU ARM virt

Provide the settings for the debug UART on the QEMU ARM virt board.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/board/emulation/qemu-arm.rst b/doc/board/emulation/qemu-arm.rst
index ca751d4..8d7fda1 100644
--- a/doc/board/emulation/qemu-arm.rst
+++ b/doc/board/emulation/qemu-arm.rst
@@ -80,3 +80,13 @@
     -drive if=none,file=disk.img,id=mydisk -device nvme,drive=mydisk,serial=foo
 
 These have been tested in QEMU 2.9.0 but should work in at least 2.5.0 as well.
+
+Debug UART
+----------
+
+The debug UART on the ARM virt board uses these settings::
+
+    CONFIG_DEBUG_UART=y
+    CONFIG_DEBUG_UART_PL010=y
+    CONFIG_DEBUG_UART_BASE=0x9000000
+    CONFIG_DEBUG_UART_CLOCK=0