dt-bindings: net: xlnx,axi-ethernet: Correct phy-mode property value

Correct phy-mode property value to 1000base-x.

Fixes: cbb1ca6d5f9a ("dt-bindings: net: xlnx,axi-ethernet: convert bindings document to yaml")
Signed-off-by: Suraj Gupta <[e-mail]>
Reviewed-by: Radhey Shyam Pandey <[e-mail]>
Reviewed-by: Andrew Lunn <[e-mail]>
Acked-by: Rob Herring (Arm) <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/devicetree/bindings/net/xlnx,axi-ethernet.yaml

···
       - gmii
       - rgmii
       - sgmii
-      - 1000BaseX
+      - 1000base-x
 
   xlnx,phy-type:
     description: