dt-bindings: net: rockchip-dwmac: add default 'input' for clock_in_out

'clock_in_out' property is optional, and it can be one of two enums.
The binding does not specify what is the behavior when the property is
missing altogether.
Hence, add a default value that the driver can use.
Signed-off-by: Eugen Hristev <eugen.hristev@collabora.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230718090914.282293-1-eugen.hristev@collabora.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -80,6 +80,7 @@ properties:
       "output" means GMAC provides the reference clock.
     $ref: /schemas/types.yaml#/definitions/string
     enum: [input, output]
+    default: input
 
   rockchip,grf:
     description: The phandle of the syscon node for the general register file.