Add a function returning currently used analogRead resolution

I'm surprised there is no function in Arduino suite that returns ADC's resolution (in bits), or maximal value that cane be read from `analogRead()`.

Having such function will allow writing a more cross-platform code that uses ADC.
For ATMega328 such function might be hard-coded to 10bit or 1023.
Other cores will return other hard-coded values or currently used ADC's resolution based on controller registers.

Search the core for the existing analogRead() implementation and related ADC configuration. Determine where a resolution or maximum-value function belongs and how the AVR core should report its value; done means the new API returns the currently used ADC resolution or equivalent maximum for supported AVR boards.

Written by the indexing model from the issue text.