This commit adds unit tests for ADC uclass's methods using sandbox ADC.

Testing proper ADC binding:
- dm_test_adc_bind()                    - device binding
- dm_test_adc_wrong_channel_selection() - checking wrong channel selection

Testing ADC supply operations:
- dm_test_adc_supply():
  - Vdd/Vss values validating
  - Vdd regulator updated value validating
  - Vdd regulator's auto enable state validating

Testing ADC operations results:
- dm_test_adc_single_channel_conversion() - single channel start/data
- dm_test_adc_single_channel_shot()       - single channel shot
- dm_test_adc_multi_channel_conversion()  - multi channel start/data
- dm_test_adc_multi_channel_shot()        - multi channel single shot

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>