dt-bindings: soc: brcm: add BCM63xx power domain binding

BCM6318, BCM6328, BCM6362 and BCM63268 SoCs have a power domain controller
to enable/disable certain components in order to save power.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>