jtag: drivers: xlnx-pcie-xvc: Add support for SWD mode.

jtag: drivers: xlnx-pcie-xvc: Add support for SWD mode.

Add support for SWD debug to the Xilinx XVC/PCIe driver.

This is possible since the device is essentially a shift-register.
So doing SWD vs JTAG is a matter of wiring things correctly on the
RTL side (use TMS for SWDI, TDO for SWDO).

The clang static checker doesn't find any new problems with this change.

Change-Id: I3959e21440cd1036769e8e56a55e601d3e4aee9a
Signed-off-by: Moritz Fischer <moritzf@google.com>
Reviewed-on: http://openocd.zylin.com/5447
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>