dt-bindings: iio: adc: add AD4691 family

dt-bindings: iio: adc: add AD4691 family

Add DT bindings for the Analog Devices AD4691 family of multichannel
SAR ADCs (AD4691, AD4692, AD4693, AD4694).

The four variants are not compatible with each other: AD4691/AD4692 have
16 analog input channels while AD4693/AD4694 have 8, and AD4691/AD4693
top out at 500 kSPS while AD4692/AD4694 reach 1 MSPS. These differences
in channel count and maximum sample rate require distinct compatible
strings so the driver can select the correct channel configuration and
rate limits.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Radu Sabau <radu.sabau@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>