Add target info to the configuration of the pads

[davidmallasen]

See #494.

• Move the mapping to the target
• Add support for the rest of the target configurations

[davidmallasen]

@davideschiavone @christophmuellerorg Can you review this?
Running make mcu-gen PADS_TARGET=tsmc_65nm will generate the hw/asic/generic_example/pnr/inputs/test.io so you can test from the inputs in the pad_cfg.hjson file.
If there are any changes that I should make, please let me know. The behavior now should be analogous to that of the python script you told me.

Once you give me the ok, I'll clean up the test files and remove the draft mode to the PR so you can merge. I'll keep it as draft for now to not merge before cleaning.