RFC RFC: dm: Add USB support

This series adds basic driver model support to USB. The intent is to permit the various subsystems (OHCI, EHCI, XHCI) to co-exist and allow any number of USB ports of different types. So far the absolute limit on the number of USB devices is only slightly relaxed.

Only USB controllers have a real driver model device. USB devices (including the hub in the controller) are not modelled as driver model devices. This is for two reasons:

- it is easier since we don't need to bind a whole lot of devices when scanning
- the two main USB devices (block devices and Ethernet) don't have driver mode support yet anyway, so it would be pointless. However the recent network RFC has encouraged me to send this.

The basic approach is to set up the driver model structures in parallel to what already exists rather than to replace them. This allows both driver model and legacy to be used for USB, although not with the same board.

So far only XHCI is supported. As an example the Exynos XHCI driver is converted to driver model.

I would appreciate comments before going further.

Caveats:
- sandbox code is incomplete and there are no tests
- 'usb stop' does not remove existing devices (I have not yet settled on the best approach)
- the usb.h changes need to be split correctly into the patches once they are final

This series is available at u-boot-dm/usb-working.