dt-bindings: power: bcm-pmb: add BCM63138 binding

PMB can be also found on bcm63xx chipsets. It uses difference device
addresses so a new binding is required.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>