dt-bindings: add binding for USBSS-DRD controller.

This patch aim at documenting USB related dt-bindings for the
Cadence USBSS-DRD controller.
Signed-off-by: Pawel Laszczak <pawell@cadence.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

Documentation/devicetree/bindings/usb/cdns-usb3.txt

+Binding for the Cadence USBSS-DRD controller
+
+Required properties:
+ - reg: Physical base address and size of the controller's register areas.
+	 Controller has 3 different regions:
+	 - HOST registers area
+	 - DEVICE registers area
+	 - OTG/DRD registers area
+ - reg-names - register memory area names:
+	"xhci" - for HOST registers space
+	"dev" - for DEVICE registers space
+	"otg" - for OTG/DRD registers space
+ - compatible: Should contain: "cdns,usb3"
+ - interrupts: Interrupts used by cdns3 controller:
+	"host" - interrupt used by XHCI driver.
+	"peripheral" - interrupt used by device driver
+	"otg" - interrupt used by DRD/OTG  part of driver
+
+Optional properties:
+ - maximum-speed : valid arguments are "super-speed", "high-speed" and
+                   "full-speed"; refer to usb/generic.txt
+ - dr_mode: Should be one of "host", "peripheral" or "otg".
+ - phys: reference to the USB PHY
+ - phy-names: from the *Generic PHY* bindings;
+	Supported names are:
+	- cdns3,usb2-phy
+	- cdns3,usb3-phy
+
+ - cdns,on-chip-buff-size : size of memory intended as internal memory for endpoints
+	buffers expressed in KB
+
+Example:
+	usb@f3000000 {
+		compatible = "cdns,usb3";
+		interrupts = <GIC_USB_IRQ 7 IRQ_TYPE_LEVEL_HIGH>,
+				<GIC_USB_IRQ  7 IRQ_TYPE_LEVEL_HIGH>,
+				<GIC_USB_IRQ  8 IRQ_TYPE_LEVEL_HIGH>;
+		interrupt-names = "host", "peripheral", "otg";
+		reg = <0xf3000000 0x10000>,	/* memory area for HOST registers */
+			<0xf3010000 0x10000>,	/* memory area for DEVICE registers */
+			<0xf3020000 0x10000>;	/* memory area for OTG/DRD registers */
+		reg-names = "xhci", "dev", "otg";
+		phys = <&usb2_phy>, <&usb3_phy>;
+		phy-names = "cdns3,usb2-phy", "cnds3,usb3-phy";
+	};