Documentation: DT: Document SMP DT nodes and properties for BCM63138

Add binding documentation for the additional nodes and properties
required to get the secondary CPU online on the BCM63138 SoC.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

Documentation/devicetree/bindings/arm/bcm/brcm,bcm63138.txt

@@ -7,3 +7,45 @@ following properties:
 Required root node property:
 
 compatible: should be "brcm,bcm63138"
+
+An optional Boot lookup table Device Tree node is required for secondary CPU
+initialization as well as a 'resets' phandle to the correct PMB controller as
+defined in reset/brcm,bcm63138-pmb.txt for this secondary CPU, and an
+'enable-method' property.
+
+Required properties for the Boot lookup table node:
+- compatible: should be "brcm,bcm63138-bootlut"
+- reg: register base address and length for the Boot Lookup table
+
+Optional properties for the primary CPU node:
+- enable-method: should be "brcm,bcm63138"
+
+Optional properties for the secondary CPU node:
+- enable-method: should be "brcm,bcm63138"
+- resets: phandle to the relevant PMB controller, one integer indicating the internal
+  bus number, and a second integer indicating the address of the CPU in the PMB
+  internal bus number.
+
+Example:
+
+	cpus {
+		cpu@0 {
+			compatible = "arm,cotex-a9";
+			reg = <0>;
+			...
+			enable-method = "brcm,bcm63138";
+		};
+
+		cpu@1 {
+			compatible = "arm,cortex-a9";
+			reg = <1>;
+			...
+			enable-method = "brcm,bcm63138";
+			resets = <&pmb0 4 1>;
+		};
+	};
+
+	bootlut: bootlut@8000 {
+		compatible = "brcm,bcm63138-bootlut";
+		reg = <0x8000 0x50>;
+	};