Add enum to list what values of maximum-speed are exceptable since
enum's get represented as strings in DT. This also allows us to
generate a code enum to correspond to the string.
We also introduce include/dt-bindings/usb/usb.h which is a hand coded
definition of the enum. We don't have a great way to generate this
right now, however it would be better if we did.
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
Kumar Gala