tcl: Add support for the Digilent Nexys Video board

* https://store.digilentinc.com/nexys-video-artix-7-fpga-trainer-board-for-multimedia-applications/
* https://reference.digilentinc.com/_media/nexys-video:nexys_video_sch.pdf



The Nexys Video board has FTDI FT2232 whose channel B is connected to
Artix-7 FPGA's JTAG pins, and can be supported by OpenOCD's
ftdi interface. Tested to be working fine on real hardware.

Change-Id: I2996166dc8c2b6c08a9390958adfcdec8fc2bd37
Signed-off-by: Rohit Singh <rohit91.2008@gmail.com>
Signed-off-by: Marc Schink <dev@zapb.de>
Reviewed-on: http://openocd.zylin.com/4364


Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>