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 <[e-mail]>
Reviewed-by: Rob Herring (Arm) <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

Documentation/devicetree/bindings/leds/common.yaml

···
       led-max-microamp.
     $ref: /schemas/types.yaml#/definitions/uint32
 
+  default-brightness:
+    description:
+      Brightness to be set if LED's default state is on. Used only during
+      initialization. If the option is not set then max brightness is used.
+    $ref: /schemas/types.yaml#/definitions/uint32
+
   panic-indicator:
     description:
       This property specifies that the LED should be used, if at all possible,