Add a function returning currently used analogRead resolution

[abetis]

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.