19516: cpu/rpx0xx: add initial ADC support r=dylad a=dylad
### Contribution description
This PR adds initial support for RP2040 ADC peripheral.
It is rather minimalist, and only use oneshot trigger to perform a single acquisition.
I've tested this PR using a potentiometer connected between GND and 3V3.
GP26, GP27, GP28 has been tested.
### Testing procedure
Select `ADC_RES_12BIT` and flash `tests/periph_adc`
`make BOARD=rpi-pico -C tests/periph_adc`
### Issues/PRs references
None.
Co-authored-by: Dylan Laduranty <dylan.laduranty@mesotic.com>
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