dt-bindings: net: document ptp_ref clk in dwmac

ptp_ref clk has been added in DT but not documented which makes yaml
validation failed:

ethernet@5800a000: clocks: [[6, 105], [6, 103], [6, 104], [6, 123],
[6, 169], [6, 112]] is too long
ethernet@5800a000: clock-names: ['stmmaceth', 'mac-clk-tx', 'mac-clk-rx',
'eth-ck', 'ptp_ref', 'ethstp'] is too long
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>

Documentation/devicetree/bindings/net/stm32-dwmac.yaml

@@ -46,17 +46,18 @@ properties:
 
   clocks:
     minItems: 3
-    maxItems: 5
+    maxItems: 6
     items:
       - description: GMAC main clock
       - description: MAC TX clock
       - description: MAC RX clock
       - description: For MPU family, used for power mode
       - description: For MPU family, used for PHY without quartz
+      - description: PTP clock
 
   clock-names:
     minItems: 3
-    maxItems: 5
+    maxItems: 6
     contains:
       enum:
         - stmmaceth
@@ -64,6 +65,7 @@ properties:
         - mac-clk-rx
         - ethstp
         - eth-ck
+        - ptp_ref
 
   st,syscon:
     $ref: "/schemas/types.yaml#/definitions/phandle-array"