USB: serial: simple: add libtransistor console

Add simple driver for libtransistor USB console.
This device is implemented in software:
https://github.com/reswitched/libtransistor/blob/development/lib/usb_serial.c



Signed-off-by: Collin May <collin@collinswebsite.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>