xilinx-dna.cfg: generic tools for reading Xilinx Device DNA

xilinx-dna.cfg: generic tools for reading Xilinx Device DNA

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

This commit adds generic support for reading the Xilinx Spartan 6 and 7
Series (Kintex, Artix, Ultrascale) Device DNA. The code is similar to
the function in fpga/xilinx-xc6s.cfg for Spartan 6 but the register
addresses are different and the logic has been simplified.

The code was not placed in xilinx-xc7.cfg. The approach of defining taps
in the same file as library code to use them is fundamentally broken on
boards that have more than one FPGA or other chips. This commit (like
the addition of support for Xilinx XADC) starts to remedy that by
splitting library code from board-specific fixed definitions.

The support code is sourced in the Kasli and KC705 board support files
as it was tested on these boards.

Change-Id: Iba559c7c1b7e93e1270535fd9e6650007f3794da
Signed-off-by: Robert Jordens <jordens@gmail.com>
Reviewed-on: http://openocd.zylin.com/4396
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>