dt-bindings: arm: Document Actions Semi S900

The Actions Semi S900 is a quad-core ARM Cortex-A53 SoC.
The Bubblegum-96 is a 96Boards Consumer Edition compliant board (4/96).

Cc: 96boards@ucrobotics.com
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>

Documentation/devicetree/bindings/arm/actions.txt

@@ -22,3 +22,18 @@ Boards:
 Root node property compatible must contain, depending on board:
 
  - LeMaker Guitar Base Board rev. B: "lemaker,guitar-bb-rev-b", "lemaker,guitar"
+
+
+S900 SoC
+========
+
+Required root node properties:
+
+- compatible :  must contain "actions,s900"
+
+
+Boards:
+
+Root node property compatible must contain, depending on board:
+
+ - uCRobotics Bubblegum-96: "ucrobotics,bubblegum-96"