Add support for AD4170 sigma-delta ADCs

[machschmitt]

PR Description

Add support for AD4170 Sigma-Delta ADC. This ADC supports many configurations. One tricky part is supporting different channel setups (differential / pseudo-differential) and voltage reference supplies. The added documentation aims to help review this driver by explaining the many possible ADC channel configurations and tries to generalize the consolidated knowledge so it may be useful for other ADCs.

PR Type

• [ ] Bug fix (a change that fixes an issue)
• [x] New feature (a change that adds new functionality)
• [ ] Breaking change (a change that affects other repos or cause CIs to fail)

PR Checklist

• [x] I have conducted a self-review of my own code changes
• [x] I have tested the changes on the relevant hardware - Tested differential channels with AD4170 on de10 nano and with AD4170 on cora. The eval-board seems to be filtering the analog input to some channels. Due to that pseudo-differential channel configurations were not tested yet.
• [ ] I have updated the documentation outside this repo accordingly (the new docs are intended to help understand this new driver)

[machschmitt]

Currently, the CI complains about not finding dt-bindings/iio/adc/adi,ad4170.h but it is being included in the first patch. Also, checkpatch check/warning is just about reusing macro parameters which is not a real issue.

[nunojsa]

Hi, I'm just checking in on the status of this pull request. Is there anything I can do to help move it forward?

[machschmitt]

Hi, I'm just checking in on the status of this pull request. Is there anything I can do to help move it forward?

No, parts of this have already or are about to go upstream. There are some features here (SPI offloading support) that may take some time to get upstreamed, so I'd like the branch not to be deleted until then.