drivers: adc: ad4114: add driver support

The AD4114 is a low power, low noise, 24-bit, sigma-delta ADC.
This driver allows to use it with the Zephyr ADC API. It uses
the continuous acquisition ADC feature.

This ADC allows many configutations, but this driver uses it as the
most generic way :
- each can channel can be enable or disable using the device
tree configuration
- configure two setups (one for unipolar inputs, one for bipolar inputs)
- use an external clock

Signed-off-by: Pierrick Curt <pierrickcurt@gmail.com>