Zephyr is still working on the plan upstream for generically controlling display "backlight" pins with GPIO/PWM, so in the meantime, add our own chosen property zmk,display-led that is set to an LED device child to allow blanking/unblanking of devices that use a dedicated backlight control pin.

Basically looks like this in the devicetree:

/ {
    chosen {
        zephyr,display = &vik_st7789v;
        zmk,display-led = &disp_led;
    };

    leds {
        compatible = "gpio-leds";

        disp_led: disp_led {
            gpios = <&vik_conn 3 GPIO_ACTIVE_HIGH>;
            label = "Display LED";
        };
    };
};

And then you need to enable the CONFIG_LED=y Kconfig flag to build that driver subsystem in.

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.