dt-bindings: pwm-backlight: Move brightness-levels to optional

The patch 'backlight: pwm_bl: compute brightness of LED linearly to
human eye' introduced a default brightness-levels table that is used
when brightness-levels is not available in the dts. So move
brightness-levels and default-brightness-level to be optional.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>