dt-bindings: iio: adc: add driver for the ti-adc084s021 chip

This adds support for the Texas Instruments ADC084S021 ADC chip.

Signed-off-by: Mårten Lindahl <martenli@axis.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>