[RP2xxx] Configure and access ADC from pins helper

[MatthiasPortzel]

The ADC integration that was in the pins helper had completely bit-rotted, so in #303 I replaced it with FIXME comments. I think the change is trivial, I'm just not at all familiar with the ADC on the RP2040. We need to replace .init() and then write a quick test program. (It's possible we don't need to replace .init and we can just delete it, but again, I don't know.)