dt-bindings: marvell: Add documentation for the Armada 3700 AVS binding

Extend the documentation of the Armada 37xx SoC with the Adaptive Voltage
Scaling (AVS) registers.
Signed-off-by: NGregory CLEMENT <gregory.clement@bootlin.com>
Acked-by: NViresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>

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

@@ -33,3 +33,18 @@ nb_pm: syscon@14000 {
 	compatible = "marvell,armada-3700-nb-pm", "syscon";
 	reg = <0x14000 0x60>;
 }
+
+AVS
+---
+
+For AVS an other component is needed:
+
+Required properties:
+- compatible     : should contain "marvell,armada-3700-avs", "syscon";
+- reg            : the register start and length for the AVS
+
+Example:
+avs: avs@11500 {
+	compatible = "marvell,armada-3700-avs", "syscon";
+	reg = <0x11500 0x40>;
+}