dt-bindings: Add arm64 ARCH_THUNDER2 platform documentation

Add documentation for Cavium's ThunderX2 CN99XX ARM64 processor. This
SoC will use "cavium,thunderx2-cn9900" as the compatible property.

Also add a documentation entry for the "cavium,thunder2" cpu core
present in these SoCs.
Signed-off-by: NJayachandran C <jnair@caviumnetworks.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NArnd Bergmann <arnd@arndb.de>

Documentation/devicetree/bindings/arm/cavium-thunder2.txt

+Cavium ThunderX2 CN99XX platform tree bindings
+----------------------------------------------
+
+Boards with Cavium ThunderX2 CN99XX SoC shall have the root property:
+  compatible = "cavium,thunderx2-cn9900", "brcm,vulcan-soc";
+
+These SoC uses the "cavium,thunder2" core which will be compatible
+with "brcm,vulcan".

Documentation/devicetree/bindings/arm/cpus.txt

@@ -170,6 +170,7 @@ nodes to be present and contain the properties described below.
 			    "brcm,brahma-b15"
 			    "brcm,vulcan"
 			    "cavium,thunder"
+			    "cavium,thunder2"
 			    "faraday,fa526"
 			    "intel,sa110"
 			    "intel,sa1100"