Generic USB Properties

Optional properties:
 - maximum-speed: tells USB controllers we want to work up to a certain
			speed. Valid arguments are "super-speed", "high-speed",
			"full-speed" and "low-speed". In case this isn't passed
			via DT, USB controllers should default to their maximum
			HW capability.
 - dr_mode: tells Dual-Role USB controllers that we want to work on a
			particular mode. Valid arguments are "host",
			"peripheral" and "otg". In case this attribute isn't
			passed via DT, USB DRD controllers should default to
			OTG.

This is an attribute to a USB controller such as:

dwc3@4a030000 {
	compatible = "synopsys,dwc3";
	reg = <0x4a030000 0xcfff>;
	interrupts = <0 92 4>
	usb-phy = <&usb2_phy>, <&usb3,phy>;
	maximum-speed = "super-speed";
	dr_mode = "otg";
};