dt-bindings: net: rockchip-dwmac: document pcs-handle

The Rockchip GMAC binding needs to describe the PCS reference used by
the SGMII support added later in this series. The property will be
parsed by rk_pcs_init(), and a missing phandle fails the probe. Add it
and require it when phy-mode is "sgmii" on rockchip,rk3568-gmac, the
only SoC in this binding that has SGMII support.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Coia Prant <coiaprant@gmail.com>
Link: https://patch.msgid.link/20261017054406.1828061-5-coiaprant@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/devicetree/bindings/net/rockchip-dwmac.yaml

@@ -120,6 +120,12 @@ properties:
     maximum: 0x7F
     default: 0x10
 
+  pcs-handle:
+    description:
+      Specifies a reference to a node representing the PCS device
+      connected to this GMAC. Required when phy-mode is "sgmii".
+    maxItems: 1
+
   phy-supply:
     description: PHY regulator
 
@@ -162,6 +168,27 @@ allOf:
         clocks:
           minItems: 5
 
+  - if:
+      properties:
+        compatible:
+          contains:
+            const: rockchip,rk3568-gmac
+    then:
+      required:
+        - phy-mode
+
+  - if:
+      properties:
+        compatible:
+          contains:
+            const: rockchip,rk3568-gmac
+        phy-mode:
+          contains:
+            const: sgmii
+    then:
+      required:
+        - pcs-handle
+
 unevaluatedProperties: false
 
 examples: