dt-bindings: iio: add AD74413R

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.

Signed-off-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20211205114045.173612-3-cosmin.tanislav@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>