dt-bindings: iio: adc: add ad7779 doc

Add dt bindings for AD7779 8-channel, simultaneous sampling ADC
family with eight full Σ-Δ ADCs on chip and ultra-low input
current to allow direct sensor connection.

Signed-off-by: Ramona Alexandra Nechita <ramona.nechita@analog.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20241014143204.30195-2-ramona.nechita@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>