doc: boards: amlogic: add documentation for BananaPi CM4IO

Add build instructions for the BananaPi BPI-CM4IO carrier board with
BPI-CM4 module.

Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Link: https://lore.kernel.org/r/20230320114609.930145-31-christianshewitt@gmail.com
[narmstrong: fixed doc build]
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
diff --git a/board/amlogic/w400/MAINTAINERS b/board/amlogic/w400/MAINTAINERS
index 96ccda2..26a4c2c 100644
--- a/board/amlogic/w400/MAINTAINERS
+++ b/board/amlogic/w400/MAINTAINERS
@@ -3,4 +3,6 @@
 S:	Maintained
 L:	u-boot-amlogic@groups.io
 F:	board/amlogic/w400/
+F:	configs/bananapi-cm4-cm4io_defconfig
 F:	doc/board/amlogic/w400.rst
+F:	doc/board/amlogic/bananapi-cm4io.rst