dt-bindings: serial: Document the graph port

A serial controller could be connected to an external connector like PCIe
M.2 for controlling the serial interface of the card. Hence, document the
OF graph port.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Link: https://patch.msgid.link/20260326-pci-m2-e-v7-4-43324a7866e6@oss.qualcomm.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>

Author: Mani-Sadhasivam

Documentation/devicetree/bindings/serial/serial.yaml

@@ -87,6 +87,9 @@ properties:
     description:
       TX FIFO threshold configuration (in bytes).
 
+  port:
+    $ref: /schemas/graph.yaml#/properties/port
+
 patternProperties:
   "^(bluetooth|bluetooth-gnss|embedded-controller|gnss|gps|mcu|onewire)$":
     if: