doc: board: apalis-imx8x: add documentation

This documents the u-boot build and deployment procedure.

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
diff --git a/doc/board/toradex/index.rst b/doc/board/toradex/index.rst
index 16b5a07..abba648 100644
--- a/doc/board/toradex/index.rst
+++ b/doc/board/toradex/index.rst
@@ -7,6 +7,7 @@
    :maxdepth: 2
 
    apalix-imx8
+   apalix-imx8x
    colibri_imx7
    colibri-imx8x
    verdin-imx8mm