Provide means to force the USB port into device, host or OTG mode at runtime

As discussed in OS-issues#129 (closed), currently the kernel doesn't provide a way to switch the USB port between device, host and OTG mode at runtime. This is a problem because USB 3 SuperSpeed can't be used in OTG mode. At present, changes to the DTB are needed to switch modes which is less than convenient. It would be useful to provide some userspace-accessible knob to switch modes.