Phy: DT binding documentation for the Armada 375 USB cluster binding

Armada 375 comes with an USB2 host and device controller and an USB3
controller. The USB cluster control register allows to manage common
features of both USB controllers. This commit adds the Device Tree
binding documentation for this piece of hardware.
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

Documentation/devicetree/bindings/phy/phy-mvebu.txt

@@ -20,3 +20,24 @@ Example:
 			#phy-cells = <0>;
 			status = "ok";
 		};
+
+Armada 375 USB cluster
+----------------------
+
+Armada 375 comes with an USB2 host and device controller and an USB3
+controller. The USB cluster control register allows to manage common
+features of both USB controllers.
+
+Required properties:
+
+- compatible: "marvell,armada-375-usb-cluster"
+- reg: Should contain usb cluster register location and length.
+- #phy-cells : from the generic phy bindings, must be 1. Possible
+values are 1 (USB2), 2 (USB3).
+
+Example:
+		usbcluster: usb-cluster@18400 {
+			compatible = "marvell,armada-375-usb-cluster";
+			reg = <0x18400 0x4>;
+			#phy-cells = <1>
+		};