Staging/max77840 device driver

[joan-na-good]

PR Description

This PR introduces initial support for the MAX77840 PMIC device driver.

The changes include:

• MFD (Multi-Function Device) parent driver
• Charger, Regulator, and Fuel Gauge sub-drivers
• YAML-based Device Tree bindings with validation

All code has been built successfully, passed checkpatch.pl in strict mode. No external dependencies are required.

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)