board: asus: grouper: implement multi-DTB support

Use board revision detection mechanism to choose correct DTB.
Adjust documentation and build setup accordingly.

Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
diff --git a/doc/board/asus/index.rst b/doc/board/asus/index.rst
index 2b10328..2cac04c 100644
--- a/doc/board/asus/index.rst
+++ b/doc/board/asus/index.rst
@@ -6,6 +6,6 @@
 .. toctree::
    :maxdepth: 2
 
-   grouper_common
+   grouper
    transformer_t20
    transformer_t30