dt-bindings: ata: sata: Document the graph port

An external connector like M.2 could expose the SATA interface to the
plugin cards. So add the graph port to establish link between the SATA
port and the connector node.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>

Author: Mani-Sadhasivam

Documentation/devicetree/bindings/ata/sata-common.yaml

@@ -54,4 +54,7 @@ $defs:
           each port can have a Port Multiplier attached thus allowing to
           access more than one drive by means of a single SATA port.
 
+      port:
+        $ref: /schemas/graph.yaml#/properties/port
+
 ...