Add constraints manipulations and message severity rules to allow multiple instantiations of the same transceiver IP

When instantiation multiple instances of the same transceiver IP core, Vivado generates critical warnings. This is ugly and distracting. Even worse: when the IP core happens to be generated for a transceiver that one actually needs, Vivado throws an error at bit file creation time.

This merge request adds constraints manipulations and message severity rules to 'hide' the above effects in plain Vivado info messages.