dt-bindings: net: add pcs-handle attribute

Document the new pcs-handle attribute to support connecting to an
external PHY. For Xilinx's AXI Ethernet, this is used when the core
operates in SGMII or 1000Base-X modes and links through the internal
PCS/PMA PHY.

Signed-off-by: Andy Chiu <[e-mail]>
Reviewed-by: Greentime Hu <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Reviewed-by: Andrew Lunn <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/devicetree/bindings/net/ethernet-controller.yaml

···
   phy-mode:
     $ref: "#/properties/phy-connection-type"
 
+  pcs-handle:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      Specifies a reference to a node representing a PCS PHY device on a MDIO
+      bus to link with an external PHY (phy-handle) if exists.
+
   phy-handle:
     $ref: /schemas/types.yaml#/definitions/phandle
     description:

Documentation/devicetree/bindings/net/xilinx_axienet.txt

···
 		  specified, the TX/RX DMA interrupts should be on that node
 		  instead, and only the Ethernet core interrupt is optionally
 		  specified here.
-- phy-handle	: Should point to the external phy device.
+- phy-handle	: Should point to the external phy device if exists. Pointing
+		  this to the PCS/PMA PHY is deprecated and should be avoided.
 		  See ethernet.txt file in the same directory.
 - xlnx,rxmem	: Set to allocated memory buffer for Rx/Tx in the hardware
 
···
  - mdio		: Child node for MDIO bus. Must be defined if PHY access is
 		  required through the core's MDIO interface (i.e. always,
 		  unless the PHY is accessed through a different bus).
+
+ - pcs-handle: 	  Phandle to the internal PCS/PMA PHY in SGMII or 1000Base-X
+		  modes, where "pcs-handle" should be used to point
+		  to the PCS/PMA PHY, and "phy-handle" should point to an
+		  external PHY if exists.
 
 Example:
 	axi_ethernet_eth: ethernet@40c00000 {