Provide means to switch USB port between device, host and OTG modes

According to the i.MX 8M Reference Manual, revision 2, section 11.1.1.1:

"Super-speed operation is not supported when OTG is enabled."

and section 11.1.1.2:

"The USB 3.0 module operates in following modes.

• Host Mode: SS/HS/FS/LS

• Device Mode: SS/HS/FS

• OTG: HS/FS/LS"

Which is quite a drawback in cases where Super-Speed would be expected, such as when the device is connected to a USB 3 hard disk or a USB 3 docking station with many peripherals.

Therefore it would useful to have some simple means to switch the USB port between host, device and OTG modes. For example, ideally a set of radio buttons in the shell's drop-down menu, or perhaps a page in GNOME Control Centre or even just a simple GUI program.

Related issue: linux-next#114