periph/adc: add adc_sample_multi()

[benpicco]

Contribution description

This adds an API to sample multiple ADC samples into a into a buffer.
Multiple ADC lines can be sampled in a round-robin fashion or in parallel if the hardware supports it.

Testing procedure

No integration in the test app yet, but I can produce some nice graphs of the sample data with Gnuplot:

two lines on a single ADC

two lines on different ADCs

Issues/PRs references

alternative to #20619

[Teufelchen1]

I think this needs to be reworded. I do not understand how many buffers I need and how to specify their size.

[benpicco]

I re-shuffled the arguments to make the compiler able to check the array sizes, I hope this also makes this easier to understand.

[Teufelchen1]

Have you tried to implement it for another board as well? Just to see if the proposed API is not over-fitted for the SAM (unlikely).

[benpicco]

@maribu suggested to make the memory layout of the samples implementation defined and provide an accessor function for individual elements.

I'm not a huge fan, since this hurts the ergonomics of the result, but if this turns out to be necessary to reach full performance on some platforms I'm willing to do it - but I'd like to avoid uglifying the API 'just in case'.