MIPS: BMIPS: add BCM6318 power domain definitions

BCM6318 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>