dt-bindings: phy: brcm,brcmstb-usb-phy: add power-domains

On BCM4908 USB PHY is managed using power controller so it needs
describing properly using the power-domains.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20210309182616.25783-1-zajec5@gmail.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>