dt-bindings: usb: add interface binding

Add a new binding for USB interface nodes, which are child nodes of
USB device nodes and addressed by interface number and configuration
value tuples.

Also add a new binding for USB combined nodes, which are special case
nodes for simple USB devices for which they replace the device and
interface nodes.

For completeness, define the already used terms "host-controller node",
"device node" and "hub node".

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>