dt-bindings: leds: pwm: Add default-brightness property

Optional default-brightness property specifies brightness value to be
used if default LED state is on.

Signed-off-by: George Stark <gnstark@salutedevices.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20241105185006.1380166-2-gnstark@salutedevices.com


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