dt-bindings: net: dwmac: document 'mac-mode' property

This change documents the 'mac-mode' property that was introduced in the
'stmmac' driver to support passive mode converters that can sit in-between
the MAC & PHY.
Signed-off-by: NAlexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

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

@@ -112,6 +112,14 @@ properties:
   reset-names:
     const: stmmaceth
 
+  mac-mode:
+    maxItems: 1
+    description:
+      The property is identical to 'phy-mode', and assumes that there is mode
+      converter in-between the MAC & PHY (e.g. GMII-to-RGMII). This converter
+      can be passive (no SW requirement), and requires that the MAC operate
+      in a different mode than the PHY in order to function.
+
   snps,axi-config:
     $ref: /schemas/types.yaml#definitions/phandle
     description: