MIPS: BMIPS: dts: add BCM63268 power domain support

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>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>