tcl/fpga/xilinx-xadc.cfg: add support for XADC

tcl/fpga/xilinx-xadc.cfg: add support for XADC

The 7 Series FPGAs contain an on-chip 12 bit ADC that can probe die
temperature, internal power supply rail voltages as well as external
voltages. The XADC is available both from fabric as well as through the
JTAG TAP.

This code implements access throught the JTAG TAP.

https://www.xilinx.com/support/documentation/user_guides/ug480_7Series_XADC.pdf

Change-Id: I6cef4d0244add71749fa28b58a736302151cc4dd
Signed-off-by: Robert Jordens <jordens@gmail.com>
Reviewed-on: http://openocd.zylin.com/4395
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>