dt-bindings: leds: Mention GPIO triggers

We reuse the trigger-sources phandle to just point to
GPIOs we may want to use as LED triggers.

Example:

gpio: gpio@0 {
    compatible "my-gpio";
    gpio-controller;
    #gpio-cells = <2>;
    interrupt-controller;
    #interrupt-cells = <2>;
    #trigger-source-cells = <2>;
};

leds {
    compatible = "gpio-leds";
    led-my-gpio {
        label = "device:blue:myled";
        gpios = <&gpio 0 GPIO_ACTIVE_HIGH>;
        default-state = "off";
        linux,default-trigger = "gpio";
        trigger-sources = <&gpio 1 GPIO_ACTIVE_HIGH>;
    };
};
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230926-gpio-led-trigger-dt-v2-2-e06e458b788e@linaro.orgSigned-off-by: Lee Jones <lee@kernel.org>

Documentation/devicetree/bindings/leds/common.yaml

@@ -191,6 +191,8 @@ properties:
       each of them having its own LED assigned (assuming they are not
       hardwired). In such cases this property should contain phandle(s) of
       related source device(s).
+      Another example is a GPIO line that will be monitored and mirror the
+      state of the line (with or without inversion flags) to the LED.
       In many cases LED can be related to more than one device (e.g. one USB LED
       vs. multiple USB ports). Each source should be represented by a node in
       the device tree and be referenced by a phandle and a set of phandle