dt-bindings: leds: backlight: ktz8866: Add reg property and update example

The kinetic,ktz8866 is a I2C driver, so add the missing reg property.
And update example to make it clear.

Signed-off-by: Jianhua Lu <lujianhua000@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20230428160246.16982-1-lujianhua000@gmail.com


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