Add support for LTC7871

[adiceline]

PR Description

• This adds driver support and device tree bindings for LTC7871 Six-Phase Buck-Boost Converter.
• Datasheet: LTC7871
• Tested on RPI4 using the following demo board: DC2886A

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)

[adiceline]

V2

DT Bindings

• Updated commit message
• Updated enable-chip-ctrl-wp description
• Renamed freq-spread-range to freq-spread-percentage, changed type to string, and updated description
• Renamed modulation-signal-freq to switching-freq-divider and updated description

Kconfig

• Updated Kconfig to reflect that LTC7871depends on both SPI and OF

Driver

• Added comment for default values
• Added of_device_id
• Added devm_regulator_register
• Added contraints for ra_ext, rb_ext, rc_ext, rd_ext, and idac_setcur_uA
• Changed ltc7871_freq_spread_percentage from unsigned int to char *
• Used struct ltc7871 *ltc7871 as argument for _ltc7871_uV_to_dac and _ltc7871_dac_to_uV instead of r1 and r2
• Fixed formatting issues

[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?