dt-bindings: Add documentation for Broadcom Vulcan

Update arm/cpus.txt to add "brcm,vulcan" CPU. Add documentation
for Broadcom Vulcan boards in arm/bcm/brcm,vulcan-soc.txt
Signed-off-by: Jayachandran C <jchandra@broadcom.com>

Documentation/devicetree/bindings/arm/bcm/brcm,vulcan-soc.txt

+Broadcom Vulcan device tree bindings
+------------------------------------
+
+Boards with Broadcom Vulcan shall have the following root property:
+
+Broadcom Vulcan Evaluation Board:
+  compatible = "brcm,vulcan-eval", "brcm,vulcan-soc";
+
+Generic Vulcan board:
+  compatible = "brcm,vulcan-soc";

Documentation/devicetree/bindings/arm/cpus.txt

@@ -167,6 +167,7 @@ nodes to be present and contain the properties described below.
 			    "arm,cortex-r5"
 			    "arm,cortex-r7"
 			    "brcm,brahma-b15"
+			    "brcm,vulcan"
 			    "cavium,thunder"
 			    "faraday,fa526"
 			    "intel,sa110"