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 <[e-mail]>
Acked-by: Rob Herring (Arm) <[e-mail]>
Link: https://lore.kernel.org/r/20240605-topic-aml[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>

Documentation/devicetree/bindings/phy/amlogic,g12a-usb2-phy.yaml

···
       Phandle to a regulator that provides power to the PHY. This
       regulator will be managed during the PHY power on/off sequence.
 
+  power-domains:
+    maxItems: 1
+
 required:
   - compatible
   - reg