Qualcomm's SPMI PMIC current ADC

QPNP PMIC current ADC (IADC) provides interface to clients to read current.
A 16 bit ADC is used for current measurements. IADC can measure the current
through an external resistor (channel 1) or internal (built-in) resistor
(channel 0). When using an external resistor it is to be described by
qcom,external-resistor-micro-ohms property.

IADC node:

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

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

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

- qcom,external-resistor-micro-ohms:
    Usage: optional
    Value type: <u32>
    Definition: Sense resister value in micro Ohm.
                If not defined value of 10000 micro Ohms will be used.

Example:
	/* IADC node */
	pmic_iadc: iadc@3600 {
		compatible = "qcom,spmi-iadc";
		reg = <0x3600 0x100>;
		interrupts = <0x0 0x36 0x0 IRQ_TYPE_EDGE_RISING>;
		qcom,external-resistor-micro-ohms = <10000>;
		#io-channel-cells  = <1>;
	};

	/* IIO client node */
	bat {
		io-channels = <&pmic_iadc  0>;
		io-channel-names = "iadc";
	};
