dt-bindings: backlight: Add brightness-levels related common properties

Both files pwm-backlight.yaml and led-backlight.yaml contain properties
in common with each other, regarding the brightness levels:

- brightness-levels
- default-brightness-level

These properties can then be moved to backlight/common.yaml.

Signed-off-by: Flavio Suligoi <f.suligoi@asem.it>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20231016150554.27144-1-f.suligoi@asem.it
Signed-off-by: Lee Jones <lee@kernel.org>