Add support for the Great Scott Gadgets Cynthion board

[antoinevg]

Note: This PR depends on: #771

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This PR adds apio support for the Great Scott Gadgets Cynthion.

Cynthion is an open-hardware, open-source multi-tool for building, analyzing and hacking USB devices.

The board features:

A Lattice Semiconductor LFE5U-12F ECP5 FPGA
Three High-Speed USB interfaces, each connected to a USB3343 PHY capable of operating at up to 480 Mbps.
Two Digilent Pmod™ Compatible I/O connectors presenting 16 high-speed FPGA user IOs.
64 Mbit HyperRAM.
32 Mbit QSPI-connected Flash RAM.
6 FPGA-connected user LEDS.
1 FPGA-connected USER button.

More info: https://www.crowdsupply.com/great-scott-gadgets/cynthion