usb: provide a device tree node to USB devices

It is possible to specify a device tree node for an USB device. This is
useful if you have a static USB setup and want to use aliases which
point to these nodes, like on the Raspberry Pi.
The nodes are matched against their hub port number, the compatible
strings are not matched for now.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>