dt-bindings: marvell: Add documentation for the North Bridge PM on Armada 37xx

Extend the documentation of the Armada 37xx SoC with the the North
Bridge Power Management component.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/devicetree/bindings/arm/marvell/armada-37xx.txt

@@ -14,3 +14,22 @@ following property before the previous one:
 Example:
 
 compatible = "marvell,armada-3720-db", "marvell,armada3720", "marvell,armada3710";
+
+
+Power management
+----------------
+
+For power management (particularly DVFS and AVS), the North Bridge
+Power Management component is needed:
+
+Required properties:
+- compatible     : should contain "marvell,armada-3700-nb-pm", "syscon";
+- reg            : the register start and length for the North Bridge
+		    Power Management
+
+Example:
+
+nb_pm: syscon@14000 {
+	compatible = "marvell,armada-3700-nb-pm", "syscon";
+	reg = <0x14000 0x60>;
+}