Currently, the links related to the modules/dsp files
do not point to the correct path.

This commit fixes the problem by adding the ".vhd"
extension to each file.

See #57

These XDC files can be used by Vivado projects to automatically infer
proper timing constraints for CDC paths.

As a generic module to update xc7 firmware.

This is necessary in order to properly "emulate" the previous implementation
of the gc_sync_ffs module.

Furthermore, a "new" module has been introduced, the gc_edge_detect, which
combines positive and negative pulse edge detection. gc_negedge and gc_posedge
have been rewritten to use internally the new gc_edge_detect.

Signed-off-by: Dimitris Lampridis <dimitris.lampridis@cern.ch>

Signed-off-by: Federico Vaga <federico.vaga@cern.ch>

Keep it consistent with all our projects: we use mainly .rst and there are
already scripts based on this assumption

Signed-off-by: Federico Vaga <federico.vaga@cern.ch>