USB EHCI controllers

Required properties:
  - compatible : should be "usb-ehci".
  - reg : should contain at least address and length of the standard EHCI
    register set for the device. Optional platform-dependent registers
    (debug-port or other) can be also specified here, but only after
    definition of standard EHCI registers.
  - interrupts : one EHCI interrupt should be described here.
If device registers are implemented in big endian mode, the device
node should have "big-endian-regs" property.
If controller implementation operates with big endian descriptors,
"big-endian-desc" property should be specified.
If both big endian registers and descriptors are used by the controller
implementation, "big-endian" property can be specified instead of having
both "big-endian-regs" and "big-endian-desc".

Example (Sequoia 440EPx):
    ehci@e0000300 {
	   compatible = "ibm,usb-ehci-440epx", "usb-ehci";
	   interrupt-parent = <&UIC0>;
	   interrupts = <1a 4>;
	   reg = <0 e0000300 90 0 e0000390 70>;
	   big-endian;
   };
