Add FTDI SWD driver

Add FTDI SWD driver

This is usable on most or all FTDI adapters using a small hardware tweak.

TCK goes to SWCLK as expected. TDO should be wired to SWDIO. For TDI there
are two options:

Either add a 74HC126 or similar tri-state buffer between TDI and SWDIO,
with OE controlled by a signal named SWDIO_OE. Or simply connect TDI and
SWDIO together via a suitable resistor (220-470 ohms or so depending on
the drive capability of the target and adapter).

nSRST (and of course Vcc, GND) may be connected too but all other signals
should be NC.

Change-Id: Id36cf4577439be96bd4e5955c3026236e1cabced
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1958
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>