net:fec: Add fsl,imx6sx-fec compatible strings

Add compatible string "fsl,imx6sx-fec" for i.MX6SX.

Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>