dt-bindings: leds: Add default-brightness property to common.yaml

dt-bindings: leds: Add default-brightness property to common.yaml

Add default-brightness property to leds/common.yaml to establish a single
canonical definition for LED brightness initialization.

The property is currently defined locally in leds/leds-pwm.yaml and is
needed by auxdisplay/titanmec,tm16xx.yaml. Properties should be defined
in only one location to avoid type inconsistencies across bindings.

Signed-off-by: Jean-François Lessard <jefflessard3@gmail.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Lee Jones <lee@kernel.org>