imx6sl fec MDIO clock source is from ipg 66Mhz, but the currect imx6sl
device tree define it as "enet_ref" clock (50Mhz), so the patch just
corrects imx6sl dtsi fec "ipg" clock.
Signed-off-by: NFugang Duan <B38611@freescale.com>
Signed-off-by: NShawn Guo <shawn.guo@freescale.com>