of: Define board compatible for MINIX NEO-X8

Document the board compatible property for MINIX NEO-X8, a
Meson8-based digital media player. While at it, move the other
existing Meson board compatible to amlogic.txt.
Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Signed-off-by: Carlo Caione <carlo@endlessm.com>

Documentation/devicetree/bindings/arm/amlogic.txt

@@ -8,3 +8,7 @@ Boards with the Amlogic Meson6 SoC shall have the following properties:
 Boards with the Amlogic Meson8 SoC shall have the following properties:
   Required root node property:
     compatible: "amlogic,meson8";
+
+Board compatible values:
+  - "geniatech,atv1200"
+  - "minix,neo-x8"

Documentation/devicetree/bindings/arm/geniatech.txt

-Geniatech platforms device tree bindings
--------------------------------------------
-
-Geniatech ATV1200
-    - compatible = "geniatech,atv1200"