docs: boards: amlogic: add documentation for BananaPi M2S

Add build docs for the BPI-M2S board.

Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20230323143142.780306-9-christianshewitt@gmail.com
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
diff --git a/doc/board/amlogic/index.rst b/doc/board/amlogic/index.rst
index 8388acd..fa1b362 100644
--- a/doc/board/amlogic/index.rst
+++ b/doc/board/amlogic/index.rst
@@ -87,6 +87,7 @@
 
    bananapi-cm4io
    bananapi-m2pro
+   bananapi-m2s
    bananapi-m5
    beelink-gskingx
    beelink-gtking