Qualcomm's SPMI PMIC voltage ADC

SPMI PMIC voltage ADC (VADC) provides interface to clients to read
voltage. The VADC is a 15-bit sigma-delta ADC.

VADC node:

- compatible:
    Usage: required
    Value type: <string>
    Definition: Should contain "qcom,spmi-vadc".

- reg:
    Usage: required
    Value type: <prop-encoded-array>
    Definition: VADC base address and length in the SPMI PMIC register map.

- #address-cells:
    Usage: required
    Value type: <u32>
    Definition: Must be one. Child node 'reg' property should define ADC
            channel number.

- #size-cells:
    Usage: required
    Value type: <u32>
    Definition: Must be zero.

- #io-channel-cells:
    Usage: required
    Value type: <u32>
    Definition: Must be one. For details about IIO bindings see:
            Documentation/devicetree/bindings/iio/iio-bindings.txt

- interrupts:
    Usage: optional
    Value type: <prop-encoded-array>
    Definition: End of conversion interrupt.

Channel node properties:

- reg:
    Usage: required
    Value type: <u32>
    Definition: ADC channel number.
            See include/dt-bindings/iio/qcom,spmi-vadc.h

- qcom,decimation:
    Usage: optional
    Value type: <u32>
    Definition: This parameter is used to decrease ADC sampling rate.
            Quicker measurements can be made by reducing decimation ratio.
            Valid values are 512, 1024, 2048, 4096.
            If property is not found, default value of 512 will be used.

- qcom,pre-scaling:
    Usage: optional
    Value type: <u32 array>
    Definition: Used for scaling the channel input signal before the signal is
            fed to VADC. The configuration for this node is to know the
            pre-determined ratio and use it for post scaling. Select one from
            the following options.
            <1 1>, <1 3>, <1 4>, <1 6>, <1 20>, <1 8>, <10 81>, <1 10>
            If property is not found default value depending on chip will be used.

- qcom,ratiometric:
    Usage: optional
    Value type: <empty>
    Definition: Channel calibration type. If this property is specified
            VADC will use the VDD reference (1.8V) and GND for channel
            calibration. If property is not found, channel will be
            calibrated with 0.625V and 1.25V reference channels, also
            known as absolute calibration.

- qcom,hw-settle-time:
    Usage: optional
    Value type: <u32>
    Definition: Time between AMUX getting configured and the ADC starting
            conversion. Delay = 100us * (value) for value < 11, and
            2ms * (value - 10) otherwise.
            Valid values are: 0, 100, 200, 300, 400, 500, 600, 700, 800,
            900 us and 1, 2, 4, 6, 8, 10 ms
            If property is not found, channel will use 0us.

- qcom,avg-samples:
    Usage: optional
    Value type: <u32>
    Definition: Number of samples to be used for measurement.
            Averaging provides the option to obtain a single measurement
            from the ADC that is an average of multiple samples. The value
            selected is 2^(value).
            Valid values are: 1, 2, 4, 8, 16, 32, 64, 128, 256, 512
            If property is not found, 1 sample will be used.

NOTE:

Following channels, also known as reference point channels, are used for
result calibration and their channel configuration nodes should be defined:
VADC_REF_625MV and/or VADC_SPARE1(based on PMIC version) VADC_REF_1250MV,
VADC_GND_REF and VADC_VDD_VADC.

Example:

	/* VADC node */
	pmic_vadc: vadc@3100 {
		compatible = "qcom,spmi-vadc";
		reg = <0x3100 0x100>;
		interrupts = <0x0 0x31 0x0 IRQ_TYPE_EDGE_RISING>;
		#address-cells = <1>;
		#size-cells = <0>;
		#io-channel-cells = <1>;
		io-channel-ranges;

		/* Channel node */
		usb_id_nopull {
			reg = <VADC_LR_MUX10_USB_ID>;
			qcom,decimation = <512>;
			qcom,ratiometric;
			qcom,hw-settle-time = <200>;
			qcom,avg-samples = <1>;
			qcom,pre-scaling = <1 3>;
		};
	};

	/* IIO client node */
	usb {
		io-channels = <&pmic_vadc VADC_LR_MUX10_USB_ID>;
		io-channel-names = "vadc";
	};