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>