Adding Machxo3D target

[tuxbotix]

Hi!
I bought a Machxo3D evaluation board a couple of years back and didn't like the diamond toolkit and lost interest.
Thanks for Litex and Migen, I managed to get it to work by following the existing Machxo3 platform definition + documentation and would like to open a PR.

Question 1:
Is it okay to define a target without a softcore?

• I don't know if this FPGA is capable enough. - managed to get --cpu-type=picorv32 --cpu-variant=minimal to fit in.
• Alternatively, the LED chaser (or KITT's LED pattern ported from Lattice) can work without the RISC core at all.

Question 2:
Not all GPIO are defined in the platform (available on the board via headers).

• What's the best practice to do this? - should the user define as they need for each application/ setup?
• or can the platform set the labels and user be able to configure when they use?