of: documentation: add bindings documentation for Meson8b

Signed-off-by: Carlo Caione <carlo@endlessm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Documentation/devicetree/bindings/arm/amlogic.txt

@@ -9,6 +9,12 @@ Boards with the Amlogic Meson8 SoC shall have the following properties:
   Required root node property:
     compatible: "amlogic,meson8";
 
+Boards with the Amlogic Meson8b SoC shall have the following properties:
+  Required root node property:
+    compatible: "amlogic,meson8b";
+
 Board compatible values:
-  - "geniatech,atv1200"
-  - "minix,neo-x8"
+  - "geniatech,atv1200" (Meson6)
+  - "minix,neo-x8" (Meson8)
+  - "tronfy,mxq" (Meson8b)
+  - "hardkernel,odroid-c1" (Meson8b)