dt-bindings: serial: imx: Document wakeup-source property

The i.MX UART can be used as a wake-up source, document the 'wakeup-source'
property as allowed property.
Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20230921192729.71259-1-marex@denx.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/fsl-imx-uart.yaml

@@ -70,6 +70,8 @@ properties:
   interrupts:
     maxItems: 1
 
+  wakeup-source: true
+
   fsl,dte-mode:
     $ref: /schemas/types.yaml#/definitions/flag
     description: |