for my custom hardware i need to drive some LED's and since the Software PWM wasn't stable enough I opted for an LED driver. I compiled a custom firmware (24.10) with the leds-pca963x kernel module and I can confirm that it's loaded. I have my PCA9635 connect to I2C on address 0x8 and can see it with i2cdetect.
I try adding the device via sys/class like:

echo pca9635 0x08 > /sys/class/i2c-dev/i2c-0/device/new_device

running desmg afterwards shows that it tried to load the driver:

[  949.521145] leds-pca963x 0-0008: Node (null) must define between 1 and 16 LEDs
[  949.528588] leds-pca963x: probe of 0-0008 failed with error -22
[  949.534638] i2c i2c-0: new_device: Instantiated device pca9635 at 0x08

However it looks like it needs the LED's defined via DTS. I never used that so I forked the onion-dts-overlay and tried to add my own but that didn't work either.

/dts-v1/;
/plugin/;

/ {
    compatible = "onion,omega2";

    fragment@0 {
        target = <&i2c0>;  // adapt to your bus if needed

        __overlay__ {
            pca9635: pca9635@08 {
                compatible = "nxp,pca9635";
                reg = <0x08>;
                #address-cells = <1>;
                #size-cells = <0>;

                led@0 {
                    label = "user:led0";
                    reg = <0>;
                };

                led@1 {
                    label = "user:led1";
                    reg = <1>;
                };
                // Add more as needed
            };
        };
    };
};

I'm not sure where to start. Ideally I'd add everything into one package for the firmware compilation but I'm lost where that should happen.

&i2c {
    pca9635@8 {
        compatible = "nxp,pca9635";
        reg = <0x08>;
        #address-cells = <1>;
        #size-cells = <0>;
        status = "okay";

        led@0 {
            label = "led0";
            reg = <0>;
            default-state = "off";
        };

        led@1 {
            label = "led1";
            reg = <1>;
            default-state = "off";
        };

        led@2 {
            label = "led2";
            reg = <2>;
            default-state = "off";
        };

        led@3 {
            label = "led3";
            reg = <3>;
            default-state = "off";
        };

        led@4 {
            label = "led4";
            reg = <4>;
            default-state = "off";
        };

        led@5 {
            label = "led5";
            reg = <5>;
            default-state = "off";
        };

        led@6 {
            label = "led6";
            reg = <6>;
            default-state = "off";
        };

        led@7 {
            label = "led7";
            reg = <7>;
            default-state = "off";
        };

        led@8 {
            label = "led8";
            reg = <8>;
            default-state = "off";
        };

        led@9 {
            label = "led9";
            reg = <9>;
            default-state = "off";
        };

        led@10 {
            label = "led10";
            reg = <10>;
            default-state = "off";
        };

        led@11 {
            label = "led11";
            reg = <11>;
            default-state = "off";
        };

        led@12 {
            label = "led12";
            reg = <12>;
            default-state = "off";
        };

        led@13 {
            label = "led13";
            reg = <13>;
            default-state = "off";
        };

        led@14 {
            label = "led14";
            reg = <14>;
            default-state = "off";
        };

        led@15 {
            label = "led15";
            reg = <15>;
            default-state = "off";
        };
    };
};

placed in the build folder

/home/thomas/openwrt-wallcontroller/target/linux/ramips/dts

and lastly adding an include to the omega .dtsi

mt7628an_onion_omega2.dtsi

#include "mt7628an.dtsi"
#include "pca9635.dtsi"
#include <dt-bindings/gpio/gpio.h>
#include <dt-bindings/input/input.h>