add adc fix from driver

[nkrackow]

Very small PR to fix the internal ADCs by setting the clock prescalers and then recalibrating. Code from the driver branch.

[ryan-summers]

This doesn't seem to have fixed CPU temperature measurement appearing to be erratic:

[nkrackow]

Hmm, let me quickly check about the other channels.

[nkrackow]

Ok, I've confirmed that exactly these changes lead to the external inputs being very accurate. Without the changes we have the same ~3% error observed before.
I tested it by just printing adc values in an infinite loop after initialization but using exactly the same code otherwise.

[nkrackow]

That means that the CPU temp readings have an additional, probably unrelated issue.

[ryan-summers]

Hmm, then perhaps the ADC clock is not the only issue with CPU measurement stability. I'll open up an issue.

[jordens]

Ack. But then we should land this.
bors r+

[bors]

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