Refactor ADC code and add Link charger current ADC support

Refactor ADC code and move board/chip-specific part to corresponding
directories.
Implement function and console command to read Link charger current.

BUG=chrome-os-partner:7527
TEST=Read EC temperature and POT input on BDS.

Change-Id: I7fafd310ea49d9b2781f10c3453f5488da29a08a
8 files changed