tcl: Support for reading "Device DNA" from Spartan 6 devices.

tcl: Support for reading "Device DNA" from Spartan 6 devices.

Most Xilinx FPGA devices contain an embedded, unique device identifier
called the "Device DNA". The identifier is nonvolatile, permanently
programmed into the FPGA, and is unchangeable providing a great serial
/ tracking number.

Debugging was done in https://github.com/timvideos/HDMI2USB/issues/36

Change-Id: Iad03eafb40887f0321a4dc22858a7c3bf37a12b3
Signed-off-by: Tim 'mithro' Ansell <mithro@mithis.com>
Reviewed-on: http://openocd.zylin.com/2960
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>