led: Add color setting and fading to LED API

[JoeHut]

In the initial PR for the LED API (#5342), an extension for RGB colors was proposed by @mbolivar. I added support for the 3-channel ON Semiconductor NCP5623 LED, which we use for driving a RGB LED. A PR for that support was proposed in #7619 together with an extension of the API for RGB LEDs and fading of the brightness that the NCP5623 also supports via I2C commands. Since the PR didn't get any feedback, maybe it makes sense to discuss here how this API for both setting the color and fading the brightness should look like.

[zephyrbot]

Hi @mbolivar-ampere, @Mani-Sadhasivam, @simonguinot,

This issue, marked as an Enhancement, was opened a while ago and did not get any traction. It was just assigned to you based on the labels. If you don't consider yourself the right person to address this issue, please re-assing it to the right person.

Please take a moment to review if the issue is still relevant to the project. If it is, please provide feedback and direction on how to move forward. If it is not, has already been addressed, is a duplicate, or is no longer relevant, please close it with a short comment explaining the reason.

@JoeHut you are also encouraged to help moving this issue forward by providing additional information and confirming this request/issue is still relevant to you.

Thanks!

[simonguinot]

The LED API has been extended with #26101 and support for multi-color LEDs has been added. The set_color API method can be used to drive an RGB LED. Support for hardware effects (such as fading, pulse, etc) via a LED API is another beast and it is not planned for now.

Since much of this issue has been implemented, let's close it as completed.