Add Support for LTC3208

[jcroleda]

PR Description

• This PR adds driver support for LTC3208 Multi-Display Driver
• Datasheet: LTC3208
• Tested on Raspberry Pi 4B using the following evaluation board: DC824A

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

[edelweiseescala]

Hello, just sharing what I've learned so far

[jcroleda]

Changelog V2:

• removed ltc3208_get_brightness
• fixed indentations
• replaced device attributes for AUX configurations in favor of devicetree properties
• cleaned define macros (removed ltc3208_low_byte_data and ltc3208_byte, used field_prep for ltc3208_high_byte_data)
• updated GPL license on leds-ltc3208.c
• updated commit message for devicetree bindings

[jcroleda]

Changelog V3:

• replaced device_property_match_string with device_property_read_string
• updated LTC3208_NUM_AUX_LEDS to 4

[nunojsa]

Again, I would just return regmap()

[jcroleda]

This is exposed to user space by brightness set and is not used in the probe function, if so should I still remove this dev_err?

[jcroleda]

Changelog V4:
leds-ltc3208.yaml

• Removed common.yaml ref from led patternproperty
• updated descriptions for leds and AUX channel configurations for clarity

leds-ltc3208.c

• fixed formatting for multiline function calls
• Adjusted defines (removed name, updated option count/limits)
• fixed error message locations to focus within probe()
• used FIELD_PREP() in update options for consistency
• replaced device_property_read_string with device_property_match_property_string to shorten aux pin configuration code in probe

[jcroleda]

Changelog V4.1:

• Adjusted tab spacing