Add OpenOCD config for DE10-Nano

[tomaz-suller]

This is a feature request to include OpenOCD configuration files to the DE10-Nano, as none of the existing files work for the board.

The board indeed has a USB Blaster II, but as reported in enjoy-digital/litex#1477 it's not possible to communicate with the softcore synthesised into the FPGA, possibly due to the ARM core integrated on the same chip, and connected to the same JTAG chain, as the Cyclone V FPGA.