caf: modules: leds: fix support GPIO-based LED devicetree definitions

[jelledevleeschouwer]

When CONFIG_CAF_LEDS_GPIO was chosen over CONFIG_CAF_LEDS_PWM, an assertion fails that checks whether a gpio-leds compatible node has either 1 or 3 child nodes to define multi-color LEDs. However, most Nordic development boards have 4 leds, which the CAF leds module does not support and hence breaks these builds. This commit adds support for up to a combination of 4 leds in what could be analogous to an RGBW LED. The changes are made as such that as little application code as possible is affected by the change. If user code wants to take full advantage of the full channel color support it will have to redefine the OFF, ON, ON_GO_OFF, BLINK, BLINK2, BREATH, and CLOCK effects since LED_NOCOLOR is only defined for 3 channels.

[NordicBuilder]

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[jelledevleeschouwer]

CC: @MarekPieta, @nordicjm, @tejlmand, @pdunaj

[MarekPieta]

Hi @jelledevleeschouwer, Thanks for the contribution.

The CAF LEDs module documentation states that:

The LEDs module assumes that a single DTS node is a separate and complete logical LED.
The LEDs module expects that the node will hold configuration of HW LEDs responsible for reproducing all required color channels.
The number of HW LEDs configured to reproduce color channels can be either one or three (either monochromatic or following the RGB order, with the red channel defined first, then the green one, then the blue one).
If only one HW LED is used for a monochromatic setting, the module converts the tri-channel color to a single value of brightness and passes it to this single HW LED.
The LED color channels are represented as child nodes of the compatible DTS node.

Ref: https://docs.nordicsemi.com/bundle/ncs-2.8.0/page/nrf/libraries/caf/leds.html#configuring_leds_in_dts

Controlling 4 monochromatic LEDs as a single RGBW LED is not consistent with the described assumptions. It would also be problematic if the monochromatic LEDs are controlled by multiple modules - for example if module A controls the first LED and module B controls the second LED for another purpose. Every module controlling any of the monochromatic LEDs would need to know what colors/effects are set to all 4 monochromatic LEDs to submit a proper LED event, because a LED event updates all color channels of a given logical LED (https://docs.nordicsemi.com/bundle/ncs-2.7.99-cs1/page/nrf/libraries/caf/caf_overview.html#c.led_event).
Maybe you could disable leds node and redefine the GPIO LED nodes in similar way as we do here: https://github.com/nrfconnect/sdk-nrf/blob/main/applications/nrf_desktop/configuration/nrf52833dk_nrf52820/app.overlay#L22-L45
See documentation for more details on how to configure GPIO LEDs used by CAF LEDs module in the DTS: https://docs.nordicsemi.com/bundle/ncs-2.8.0/page/nrf/libraries/caf/leds.html#configuring_gpio_leds

Would this approach be acceptable for your use-case?