USB xHCI controllers

Required properties:
  - compatible: should be one of "generic-xhci",
    "marvell,armada-375-xhci", "marvell,armada-380-xhci",
    "renesas,xhci-r8a7790", "renesas,xhci-r8a7791" (deprecated:
    "xhci-platform").
  - reg: should contain address and length of the standard XHCI
    register set for the device.
  - interrupts: one XHCI interrupt should be described here.

Optional property:
  - clocks: reference to a clock

Example:
	usb@f0931000 {
		compatible = "generic-xhci";
		reg = <0xf0931000 0x8c8>;
		interrupts = <0x0 0x4e 0x0>;
	};
