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

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";

	};