dm: adc: add simple ADC uclass implementation

This commit adds:
- new uclass id: UCLASS_ADC
- new uclass driver: drivers/adc/adc-uclass.c

The new uclass's API allows for ADC operation on:
* single-

dm: adc: add simple ADC uclass implementation

This commit adds:
- new uclass id: UCLASS_ADC
- new uclass driver: drivers/adc/adc-uclass.c

The new uclass's API allows for ADC operation on:
* single-channel with channel selection by a number
* multti-channel with channel selection by bit mask

ADC uclass's functions:
* single-channel:
- adc_start_channel() - start channel conversion
- adc_channel_data() - get conversion data
- adc_channel_single_shot() - start/get conversion data
* multi-channel:
- adc_start_channels() - start selected channels conversion
- adc_channels_data() - get conversion data
- adc_channels_single_shot() - start/get conversion data for channels
selected by bit mask
* general:
- adc_stop() - stop the conversion
- adc_vdd_value() - positive reference Voltage value with polarity [uV]
- adc_vss_value() - negative reference Voltage value with polarity [uV]
- adc_data_mask() - conversion data bit mask

The device tree can provide below constraints/properties:
- vdd-polarity-negative: if true: Vdd = vdd-microvolts * (-1)
- vss-polarity-negative: if true: Vss = vss-microvolts * (-1)
- vdd-supply: phandle to Vdd regulator's node
- vss-supply: phandle to Vss regulator's node
And optional, checked only if the above corresponding, doesn't exist:
- vdd-microvolts: positive reference Voltage [uV]
- vss-microvolts: negative reference Voltage [uV]

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

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