doc: opendous interface based on ft2232H

It was listed in the ft2232 based cables supported. Moreover, the
ft2232_channel option, which was added to support this cable, was explained.

Change-Id: I82ebc7bc10d6472f96ab150e78d623a617edccd2
Signed-off-by: Rodrigo Melo <rmelo@inti.gob.ar>
Reviewed-on: http://openocd.zylin.com/1098
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>