ADC device binding

There are no mandatory properties for ADC. However, if Voltage info is required,
then there are two options:
- use microvolts constraint or
- use regulator phandle to enable/read supply's Voltage

Properties and constraints:
*optional and always checked, Voltage polarity info:
- vdd-polarity-negative:  positive reference Voltage has a negative polarity
- vss-polarity-negative:  negative reference Voltage has a negative polarity

Chose one option, for each supply (Vdd/Vss):

*optional and always checked, supply Voltage constants:
- vdd-supply:            phandle to Vdd regulator's node
- vss-supply:            phandle to Vss regulator's node

*optional and checked only if the above corresponding, doesn't exist:
- vdd-microvolts:        positive reference Voltage value [uV]
- vss-microvolts:        negative reference Voltage value [uV]

Example with constant 'Vdd' value:
adc@1000000 {
	compatible = "some-adc";
	reg = <0xaabb000 0x100>;
	status = "enabled";
	vdd-microvolts = <1800000>;
};

Example of supply phandle usage, for the ADC's VDD/VSS references as below:
   _______         _______
  |Sandbox|       |Sandbox|
  : PMIC  :       :  ADC  :
  .       .       .       .
  |       | (Vdd) |   AIN0|-->
  |  BUCK2|-------|VDDref |
  | (3.3V)|      _|VSSref |
  |_______|     | |_______|
               _|_

For the above PMIC, the node can be defined as follows:
sandbox_pmic {
	compatible = "sandbox,pmic";
	...
	buck2: buck2 {
		regulator-name = "SUPPLY_3.3V";
		regulator-min-microvolt = <3300000>;
		regulator-max-microvolt = <3300000>;
	};
	...
};

For the above ADC, the node can be defined as follows:
adc@0 {
	compatible = "sandbox,adc";
	vdd-supply = <&buck2>;
	vss-microvolts = <0>;
};

The ADC uclass code, will enable the supply before start of the conversion,
but it will not configure the regulator settings.