dt-bindings: add SMP enable-method for Broadcom NSP

Add a compatible string "brcm,bcm-nsp-smp" for Broadcom's
Northstar Plus CPU to the 32-bit ARM CPU device tree binding
documentation file and create a new binding documentation for
Northstar Plus CPU.
Signed-off-by: Kapil Hali <kapilh@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

Documentation/devicetree/bindings/arm/bcm/brcm,nsp-cpu-method.txt

+Broadcom Northstar Plus SoC CPU Enable Method
+---------------------------------------------
+This binding defines the enable method used for starting secondary
+CPU in the following Broadcom SoCs:
+  BCM58522, BCM58525, BCM58535, BCM58622, BCM58623, BCM58625, BCM88312
+
+The enable method is specified by defining the following required
+properties in the corresponding secondary "cpu" device tree node:
+  - enable-method = "brcm,bcm-nsp-smp";
+  - secondary-boot-reg = <...>;
+
+The secondary-boot-reg property is a u32 value that specifies the
+physical address of the register which should hold the common
+entry point for a secondary CPU. This entry is cpu node specific
+and should be added per cpu. E.g., in case of NSP (BCM58625) which
+is a dual core CPU SoC, this entry should be added to cpu1 node.
+
+
+Example:
+	cpus {
+		#address-cells = <1>;
+		#size-cells = <0>;
+
+		cpu0: cpu@0 {
+			device_type = "cpu";
+			compatible = "arm,cortex-a9";
+			next-level-cache = <&L2>;
+			reg = <0>;
+		};
+
+		cpu1: cpu@1 {
+			device_type = "cpu";
+			compatible = "arm,cortex-a9";
+			next-level-cache = <&L2>;
+			enable-method = "brcm,bcm-nsp-smp";
+			secondary-boot-reg = <0xffff042c>;
+			reg = <1>;
+		};
+	};

Documentation/devicetree/bindings/arm/cpus.txt

@@ -190,6 +190,7 @@ nodes to be present and contain the properties described below.
 			    "allwinner,sun6i-a31"
 			    "allwinner,sun8i-a23"
 			    "arm,psci"
+			    "brcm,bcm-nsp-smp"
 			    "brcm,brahma-b15"
 			    "marvell,armada-375-smp"
 			    "marvell,armada-380-smp"