dt-bindings: usb: usb-nop-xceiv: add wakeup-source property

USB phy may be a system wakeup source, so add wakeup source property
to keep its resource (e.g. power domain) active to make USB remote
wakeup work.

Signed-off-by: Li Jun <[e-mail]>
Acked-by: Krzysztof Kozlowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/usb/usb-nop-xceiv.yaml

···
       the VBus line.
     $ref: /schemas/types.yaml#/definitions/phandle
 
+  wakeup-source:
+    description:
+      Specify if the USB phy can detect the remote wakeup signal
+      while the system sleep.
+
 required:
   - compatible
   - '#phy-cells'