USB xHCI controllers

Required properties:
  - compatible: should be "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.

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