stmhal - fixed up adc stuff

Added support for the ADC channels and mappings to make_pins.py

I'm not sure if the hal properly deals with the channel 16/18 differences
between the 40x and 42x. It seems to deal with it partially. This particular
aspect will need testing on a 42x or 43x.
stmhal: Make pyb.ADC the ADC class (not a function).