doc: boards: amlogic: add documentation for GS-King-X

GS-King-X is also supported with the beelink-s922x FIP sources and can use
the GT-King defconfig. Add a board document with instructions.

Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Link: https://lore.kernel.org/r/20230320114609.930145-33-christianshewitt@gmail.com
[narmstrong: fixed doc build]
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
diff --git a/doc/board/amlogic/index.rst b/doc/board/amlogic/index.rst
index 808c857..7c4c161 100644
--- a/doc/board/amlogic/index.rst
+++ b/doc/board/amlogic/index.rst
@@ -87,6 +87,7 @@
 
    bananapi-cm4io
    bananapi-m5
+   beelink-gskingx
    beelink-gtking
    beelink-gtkingpro
    jethub-j80