doc: debug UART for RISC-V QEMU virt machine

Provide settings for enabling the debug UART of the virt machine on
RISC-V QEMU.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/board/emulation/qemu-riscv.rst b/doc/board/emulation/qemu-riscv.rst
index ae99881..782f372 100644
--- a/doc/board/emulation/qemu-riscv.rst
+++ b/doc/board/emulation/qemu-riscv.rst
@@ -132,3 +132,13 @@
     -device ide-hd,drive=mydisk,bus=ahci.0
 
 You will have to run 'scsi scan' to use it.
+
+Debug UART
+----------
+
+The following settings provide a debug UART for the virt machine::
+
+    CONFIG_DEBUG_UART=y
+    CONFIG_DEBUG_UART_NS16550=y
+    CONFIG_DEBUG_UART_BASE=0x10000000
+    CONFIG_DEBUG_UART_CLOCK=3686400