dt-bindings: net: snps,dwmac: Document stmmac-axi-config subnode

The stmmac-axi-config subnode is present in multiple dwmac instance DTs,
document its content per snps,axi-config property description which is
a phandle to this subnode.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220927012449.698915-1-marex@denx.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/devicetree/bindings/net/snps,dwmac.yaml

@@ -302,6 +302,60 @@ properties:
     required:
       - compatible
 
+  stmmac-axi-config:
+    type: object
+    unevaluatedProperties: false
+    description:
+      AXI BUS Mode parameters.
+
+    properties:
+      snps,lpi_en:
+        $ref: /schemas/types.yaml#/definitions/flag
+        description:
+          enable Low Power Interface
+
+      snps,xit_frm:
+        $ref: /schemas/types.yaml#/definitions/flag
+        description:
+          unlock on WoL
+
+      snps,wr_osr_lmt:
+        $ref: /schemas/types.yaml#/definitions/uint32
+        description:
+          max write outstanding req. limit
+
+      snps,rd_osr_lmt:
+        $ref: /schemas/types.yaml#/definitions/uint32
+        description:
+          max read outstanding req. limit
+
+      snps,kbbe:
+        $ref: /schemas/types.yaml#/definitions/uint32
+        description:
+          do not cross 1KiB boundary.
+
+      snps,blen:
+        $ref: /schemas/types.yaml#/definitions/uint32-array
+        description:
+          this is a vector of supported burst length.
+        minItems: 7
+        maxItems: 7
+
+      snps,fb:
+        $ref: /schemas/types.yaml#/definitions/flag
+        description:
+          fixed-burst
+
+      snps,mb:
+        $ref: /schemas/types.yaml#/definitions/flag
+        description:
+          mixed-burst
+
+      snps,rb:
+        $ref: /schemas/types.yaml#/definitions/flag
+        description:
+          rebuild INCRx Burst
+
 required:
   - compatible
   - reg