AD8366 Driver upstream update

[rodrigo455]

PR Description

This series modernizes the AD8366 amplifier/attenuator driver and adds support for additional device variants.

• Adds support for HMC271 (legacy/obsolete part maintained in ADI tree)
• Adds support for multiple digital step attenuators:
• ADRF5720: 0.5 dB LSB, 6-Bit, Digital Attenuator, 9 kHz to 40 GHz
• ADRF5730: 0.5 dB LSB, 6-Bit, Digital Attenuator, 100 MHz to 40 GHz
• ADRF5731: 2 dB LSB, 4-Bit, Digital Attenuator, 100 MHz to 40 GHz
• HMC1018A: 1.0 dB LSB GaAs MMIC 5-BIT DIGITAL ATTENUATOR, 0.1 - 30 GHz
• HMC1019A: 0.5 dB LSB GaAs MMIC 5-BIT DIGITAL ATTENUATOR, 0.1 - 30 GHz

Device Tree Support:

• Adds device tree binding documentation
• Adds device tree compatible strings for all supported devices

PR Type

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

PR Checklist

• I have conducted a self-review of my own code changes
• I have compiled my changes, including the documentation
• I have tested the changes on the relevant hardware
• I have updated the documentation outside this repo accordingly
• I have provided links for the relevant upstream lore

[nunojsa]

You should still add your sob tag for Michael's patches. If you actually change them in any meaningful way you should add co-authored-by tag. See git log for examples

[nunojsa]

Just some minor stuff. Feel free to send the patches

[nunojsa]

Not really related to the change but on the commit message replace: s/fragment/entry

[nunojsa]

Could mention this change in the commit message

[nunojsa]

Note that given that you're doing the above, you might be asked to ditch the array in favor of dedicated variables (that is the preferred way in IIO).