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";
	};