dt-bindings: leds: backlight: Add Kinetic KTZ8866 backlight

Add Kinetic KTZ8866 backlight binding documentation.

Signed-off-by: Jianhua Lu <lujianhua000@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230120155018.15376-1-lujianhua000@gmail.com