Some boards have more than one USB port. In that case, there is a second
GPIO on the debug board to simulate a USB disconnection for those.
Add the option to describe both USB disconnect GPIOs using
"usb0_disconnect" and "usb1_disconnect" and change both of them
when disabling USB power.
In the future it might be useful to control them separately, but for now
this ensures that power to the board is cut properly when the board is
powered off.
