cpu/atmega_common: add adc driver

boards/arduino-atmega-common/Makefile.features

boards/arduino-atmega-common/include/arduino_board.h

boards/arduino-atmega-common/include/arduino_pinmap.h

boards/arduino-atmega-common/include/periph_conf.h

boards/arduino-due/include/arduino_board.h

boards/arduino-mkr-common/include/arduino_board.h

boards/arduino-mkr-common/include/arduino_pinmap.h

boards/arduino-zero/include/arduino_board.h

boards/arduino-zero/include/arduino_pinmap.h

boards/nucleo-common/include/arduino_board.h

boards/nucleo-common/include/arduino_pinmap.h

boards/nucleo144-common/include/arduino_board.h

boards/nucleo144-common/include/arduino_pinmap.h

boards/nucleo32-common/include/arduino_board.h

boards/nucleo32-common/include/arduino_pinmap.h

boards/stm32f4discovery/include/arduino_board.h

boards/stm32f4discovery/include/arduino_pinmap.h

boards/waspmote-pro/Makefile.features

boards/waspmote-pro/include/periph_conf.h

cpu/atmega_common/periph/adc.c