dt-bindings: leds: backlight: apple,dwi-bl: Add Apple DWI backlight

Add backlight controllers attached via Apple DWI 2-wire interface.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: "Daniel Thompson (RISCstar)" <danielt@kernel.org>
Signed-off-by: Nick Chan <towinchenmi@gmail.com>
Reviewed-by: Neal Gompa <neal@gompa.dev>
Link: https://lore.kernel.org/r/20250214040306.16312-2-towinchenmi@gmail.com


Signed-off-by: Lee Jones <lee@kernel.org>