Backport AD4000 series driver with offload support

[machschmitt]

PR Description

Ports back the AD4000 driver into ADI Linux with offload support as requested at https://baylibre.atlassian.net/browse/ADI-369.

0. Cherry-picks upstream commits that conflicted with SPI MOSI idle feature.
1. Ports the SPI MOSI idle support from upstream into ADI Linux.
2. Removes the ad400x driver
3. Add device tree doc for AD4000 series
4. Updates existing device tree of AD4000 series devices
5. Adds the ad4000 driver from upstream
6. Adapts the ad4000 driver to ADI Linux avoiding cherry-picking additional commits from upstream
7. Update ADI imply for AD4000
8. Add support for adjustable sampling rate

Some commits for the dts files are very similar and may be squashed. The commits adapting the ad4000 driver could also be squashed (arguably). I decided to leave them separate for now so it's more clear what the changes are. Also, squashing is easier than splitting.

Tested with ADAQ4003 on Cora and with AD7687 on Cora. Plan is to support single-channel pulsar (from ad_pulsar.c driver) in ad4000.c in the future.

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
• [ ] I have updated the documentation outside this repo accordingly (if there is the case)