tcl/interface/ftdi: improve minimodule config

tcl/interface/ftdi: improve minimodule config

- Tested on a real FT2232H MiniModule, so warning removed.
- Every pin initially set to high impedance except TCK, TDI,
  TDO and TMS: Safest values given it's an evaluation board
  and the rest of pins might be connected to something else.
- Reset is now initially de-asserted (it was asserted
  which is not recommended).
- nRST pin choice is arbitrary so comment added (wondering
  if it should be an "echo").
- "-oe" option added to NRST signal so it can be set as
  high impedance (tri-stated).

Change-Id: I967ab0c7bbccf72dbf6d6d78b3180b74e016e0d6
Signed-off-by: Diego Herranz <diegoherranz@diegoherranz.com>
Reviewed-on: http://openocd.zylin.com/4185
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>