platforms/marblemini: review/cleanup.
platforms/marblemini: review/cleanup.

- add copyrights.
- add link to documentation.
- reindent code.
- rename some IOs (for consistency with the others platforms).
- add pullup on clk20_vcxo_en.
- set clk20_vcxo as default clock.
- remove ununnecessary eth constraints. (we are using IO primitives).
- move PMODS to connectors (for consistency with others platforms).

litex-hub/litex-boards