jtag: drivers: xlnx-pcie-xvc: Add support for Xilinx XVC/PCIe

jtag: drivers: xlnx-pcie-xvc: Add support for Xilinx XVC/PCIe

Add support for Xilinx Virtual Cable over PCIe JTAG controller.
It is commonly used in Xilinx based PCI Express designs with JTAG IP
in the FPGA fabric.

Access to the JTAG registers happens via the PCI Express extended
configuration space.

This can be used to debug soft-cores instantiated in the FPGA fabric.

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

Change-Id: Ib12ede0d1f26dacfda808d5e05b947b640c5bde7
Signed-off-by: Moritz Fischer <moritzf@google.com>
Reviewed-on: http://openocd.zylin.com/5314
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Marex
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>