dt-bindings: phy: g12a-usb2-phy: add optional power-domains

On newer SoCs, the USB2 PHY can require a power-domain to operate,
add it as optional.

Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20240605-topic-amlogic-upstream-bindings-fixes-power-domains-phy-v1-1-c819b0ecd8c8@linaro.org


Signed-off-by: Vinod Koul <vkoul@kernel.org>