arm64, thunder: Document devicetree bindings for Cavium Thunder SoC

This patch adds documentation for the devicetree bindings used by the
DT files of Cavium Thunder SoC platforms.
Signed-off-by: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>
Signed-off-by: Robert Richter <rrichter@cavium.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Documentation/devicetree/bindings/arm/cavium-thunder.txt

+Cavium Thunder platform device tree bindings
+--------------------------------------------
+
+Boards with Cavium's Thunder SoC shall have following properties.
+
+Root Node
+---------
+Required root node properties:
+
+  - compatible = "cavium,thunder-88xx";

Documentation/devicetree/bindings/arm/cpus.txt

@@ -166,6 +166,7 @@ nodes to be present and contain the properties described below.
 			    "arm,cortex-r5"
 			    "arm,cortex-r7"
 			    "brcm,brahma-b15"
+			    "cavium,thunder"
 			    "faraday,fa526"
 			    "intel,sa110"
 			    "intel,sa1100"