drivers: adc: add ADC emulator

ADC emulator is designed to be used in tests on native_posix board. It
supports 1-16 bit resolution range and all GAINs from enum adc_gain.
Reference voltages and number of emulated channels are set through dts.
Using special API from drivers/adc/adc_emul.h it is possible to set
constant voltage value returned by given ADC channel or set custom
function which allows to simulate complex output.
Also reference voltages can be changed in runtime using the API.

The CL also includes:
- Add adc definitions of ADC emulator in
  tests/drivers/adc/adc_api/src/test_adc.c for supporting test suites.
- Add test for ADC emulator API in tests/drivers/adc/adc_emul/

Signed-off-by: Tomasz Michalec <tm@semihalf.com>