As found on the NI Project Sulfur SDR board.

Change-Id: I47bdd38ae85cf45cedad8797ea03bf3105153320
Signed-off-by: Moritz Fischer <moritz.fischer@ettus.com>
Reviewed-on: http://openocd.zylin.com/4176


Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>