doc: Add documentation for the ftdi driver

Change-Id: I1ade2eb187b404141051d9f59ba06e8e6e5d51aa
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1099
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>