dt-bindings: leds: add Broadcom's BCM63138 controller

Broadcom used 2 LEDs hardware blocks for their BCM63xx SoCs:
1. Older one (BCM6318, BCM6328, BCM6362, BCM63268, BCM6838)
2. Newer one (BCM6848, BCM6858, BCM63138, BCM63148, BCM63381, BCM68360)

The newer one was also later also used on BCM4908 SoC.

Old block is already documented in the leds-bcm6328.yaml. This binding
documents the new one which uses different registers & programming. It's
first used in BCM63138 thus the binding name.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>