doc: riscv: debug UART for MAIX

Provide the required settings for the debug UART.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
diff --git a/doc/board/sipeed/maix.rst b/doc/board/sipeed/maix.rst
index c235cb4..b1894f3 100644
--- a/doc/board/sipeed/maix.rst
+++ b/doc/board/sipeed/maix.rst
@@ -262,6 +262,16 @@
    stage.
 8. The boot hart jumps to ``0x80000000``.
 
+Debug UART
+^^^^^^^^^^
+
+The Debug UART is provided with the following settings::
+
+    CONFIG_DEBUG_UART=y
+    CONFIG_DEBUG_UART_SIFIVE=y
+    CONFIG_DEBUG_UART_BASE=0x38000000
+    CONFIG_DEBUG_UART_CLOCK=390000000
+
 Resetting the board
 ^^^^^^^^^^^^^^^^^^^