Your ASCII art of the chip might be useful here as documentation.

+  # We can't allOf reference Ethernet-controller.yaml because we end up with
+  # contradictory $nodename rules (`ethernet@` versus `pci@`). Happily only a
+  # small number of the properties are useful on TC956x so we can just reference
+  # what we need.

Why not add an subnodes for the ethernet interfaces?

+examples:
+  - |
+    pcie {
+      #address-cells = <3>;
+      #size-cells = <2>;
+
+      tc956x_emac0: pci@0,0 {
+        compatible = "pci1179,0220";
+        reg = <0x50000 0x0 0x0 0x0 0x0>;
+        #address-cells = <3>;
+        #size-cells = <2>;
+        device_type = "pci";
+        ranges;
+
+        gpio-controller;
+        #gpio-cells = <2>;

I've not got to the GPIO driver patch yet...

Is the GPIO part of the ethernet device, or part of the chip? The
hierarchy here should match the hierarchy of the hardware.

+        phy-mode = "10gbase-r";
+        phy-handle = <&tc956x_emac0_phy>;
+
+        mdio {
+          compatible = "snps,dwmac-mdio";
+          #address-cells = <1>;
+          #size-cells = <0>;
+
+          tc956x_emac0_phy: ethernet-phy@1c {
+            compatible = "ethernet-phy-id311c.1c12";
+            reg = <0x1c>;
+          };
+        };
+      };
+      pci@0,1 {
+        compatible = "pci1179,0220";
+        reg = <0x50100 0x0 0x0 0x0 0x0>;
+        #address-cells = <3>;
+        #size-cells = <2>;
+        device_type = "pci";
+        ranges;
+

You second ethernet does not have a gpio controller?

	Andrew