net: fec: phy-reset-gpios is optional

The phy-reset-gpios is an optional property for fec device tree boot.
Change the binding document to match the driver code.

Signed-off-by: Shawn Guo <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/devicetree/bindings/net/fsl-fec.txt

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 - phy-mode : String, operation mode of the PHY interface.
   Supported values are: "mii", "gmii", "sgmii", "tbi", "rmii",
   "rgmii", "rgmii-id", "rgmii-rxid", "rgmii-txid", "rtbi", "smii".
-- phy-reset-gpios : Should specify the gpio for phy reset
 
 Optional properties:
 - local-mac-address : 6 bytes, mac address
+- phy-reset-gpios : Should specify the gpio for phy reset
 
 Example: