ADC module lacks simple useful #defines

[Kucmasz]

It would be pretty useful to have a defines for ADC module that state

• max reading in bits
• reference voltage

for actual reading conversion in applications. Also a simple macro for converting a GPIO pin to ADC channel could be implemented easily.

[vkottler]

reference voltage

@Kucmasz this depends on hardware external to the RP2040 (ADC_AVDD). The datasheet plainly states that the ADC is 12-bit.

In practice there are often voltage dividers and other signal conditioning circuitry that would go from some application signal to the ADC input, so it doesn't seem that useful to have #define macros for this because:

1. #define is horrible
2. everything ADC related in firmware is application specific