iio: addac: add AD74413R driver

iio: addac: add AD74413R driver

The AD74412R and AD74413R are quad-channel, software configurable,
input/output solutions for building and process control applications.

They contain functionality for analog output, analog input, digital input,
resistance temperature detector, and thermocouple measurements integrated
into a single chip solution with an SPI interface.

The devices feature a 16-bit ADC and four configurable 13-bit DACs to
provide four configurable input/output channels and a suite of diagnostic
functions.

The AD74413R differentiates itself from the AD74412R by being
HART-compatible.

When configured with channel 0 as voltage output, channel 1 as current
output, channel 2 as voltage input and channel 3 as current input, the
following structure is created under the corresponding IIO device.

.
├── in_current0_offset
├── in_current0_raw
├── in_current0_sampling_frequency
├── in_current0_sampling_frequency_available
├── in_current0_scale
├── in_voltage1_offset
├── in_voltage1_raw
├── in_voltage1_sampling_frequency
├── in_voltage1_sampling_frequency_available
├── in_voltage1_scale
├── in_voltage2_offset
├── in_voltage2_raw
├── in_voltage2_sampling_frequency
├── in_voltage2_sampling_frequency_available
├── in_voltage2_scale
├── in_current3_offset
├── in_current3_raw
├── in_current3_sampling_frequency
├── in_current3_sampling_frequency_available
├── in_current3_scale
├── out_voltage0_raw
├── out_voltage0_scale
├── out_current1_raw
├── out_current1_scale
├── name
├── buffer
│   ├── data_available
│   ├── enable
│   ├── length
│   └── watermark
└── scan_elements
    ├── in_current0_en
    ├── in_current0_index
    ├── in_current0_type
    ├── in_voltage1_en
    ├── in_voltage1_index
    ├── in_voltage1_type
    ├── in_voltage2_en
    ├── in_voltage2_index
    ├── in_voltage2_type
    ├── in_current3_en
    ├── in_current3_index
    └── in_current3_type

Signed-off-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20211205114045.173612-4-cosmin.tanislav@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>