zephyr: Remove reg devicetree property from IIO devices

[petejohanson-adi]

PR Description

Simplify the devicetree syntax for IIO devices by removing the unused/unneeded reg property requirement, since the reg value is no longer used by the Zephyr backend.

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 commented new code, particularly complex or unclear areas
• I have checked that I did not introduce new warnings or errors (CI output)
• I have checked that components that use libiio did not get broken
• I have updated the documentation accordingly (GitHub Pages, READMEs, etc)