dt-bindings: arm: actions: Add CubieBoard6

Define a compatible string for Cubietech CubieBoard6.

Cc: support@cubietech.com
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NAndreas Färber <afaerber@suse.de>

Documentation/devicetree/bindings/arm/actions.txt

@@ -21,6 +21,7 @@ Boards:
 
 Root node property compatible must contain, depending on board:
 
+ - Cubietech CubieBoard6: "cubietech,cubieboard6"
  - LeMaker Guitar Base Board rev. B: "lemaker,guitar-bb-rev-b", "lemaker,guitar"