powerpc/fsl/dts: Fix phy-connection-type for fm1mac3

Property phy-connection-type contains invalid value "sgmii-2500" per scheme
defined in file ethernet-controller.yaml.

Correct phy-connection-type value should be "2500base-x".
Signed-off-by: Pali Rohár <pali@kernel.org>
Fixes: 84e0f1c1 ("powerpc/mpc85xx: Add MDIO bus muxing support to the board device tree(s)")
Acked-by: Scott Wood <oss@buserror.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/powerpc/boot/dts/fsl/t1023rdb.dts

@@ -154,7 +154,7 @@ fm1mac2: ethernet@e2000 {
 
 			fm1mac3: ethernet@e4000 {
 				phy-handle = <&sgmii_aqr_phy3>;
-				phy-connection-type = "sgmii-2500";
+				phy-connection-type = "2500base-x";
 				sleep = <&rcpm 0x20000000>;
 			};