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

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 <suraj.gupta2@amd.com>
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@amd.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20241028091214.2078726-1-suraj.gupta2@amd.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>