Documentation: DT: Add Broadcom BCM63138 PMB binding

Add a Device Tree binding for the Broadcom BCM63138 Processor Monitor
Bus, which is an internal bus used to access different power and reset
signals within a BCM63138 System-on-a-Chip.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

Documentation/devicetree/bindings/reset/brcm,bcm63138-pmb.txt

+Broadcom BCM63138 Processor Monitor Bus binding
+===============================================
+
+Please also refer to reset.txt in this directory for common reset
+controller binding usage.
+
+Require properties:
+
+- compatible: must be "brcm,bcm63138-pmb"
+- reg: base register address and size for this bus controller
+- #reset-cells: must be 2 first cell is the address within the bus instance designated
+  by the phandle, and the second is the number of zones for this peripheral
+
+Example:
+	pmb0: reset-controller@4800c0 {
+		compatible = "brcm,bcm63138-pmb";
+		reg = <0x4800c0 0x10>;
+		#reset-cells = <2>;
+	};