dt-bindings: net: phy: Document support for external PHY clk

Ethern PHYs can have external an clock that needs to be activated before
communicating with the PHY.
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -93,6 +93,12 @@ properties:
       the turn around line low at end of the control phase of the
       MDIO transaction.
 
+  clocks:
+    maxItems: 1
+    description:
+      External clock connected to the PHY. If not specified it is assumed
+      that the PHY uses a fixed crystal or an internal oscillator.
+
   enet-phy-lane-swap:
     $ref: /schemas/types.yaml#/definitions/flag
     description: