XDC parser enchancements

[litghost]

The current FPGA interchange XDC parser doesn't support the set of features required. Many of the XDC commands implemented in https://github.com/SymbiFlow/yosys-symbiflow-plugins/ need to be added to the FPGA interchange XDC parser.

Specific examples:

• [get_ports] should return all ports
• None of the clock commands are implemented.

Linux LiteX XDC file is a good driving example.

[kowalewskijan]

This issue is related to the: chipsalliance/fpga-tool-perf#323

The above PR is blocked as we miss e.g.:

• create_clock command
• LOC keyword support (interchange: allow LOC keyword in XDC files YosysHQ/nextpnr#683)