ARM: dts: imx6sl: correct the fec ipg clock source

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: Fugang Duan <B38611@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>

arch/arm/boot/dts/imx6sl.dtsi

@@ -686,7 +686,7 @@ fec: ethernet@02188000 {
 				compatible = "fsl,imx6sl-fec", "fsl,imx25-fec";
 				reg = <0x02188000 0x4000>;
 				interrupts = <0 114 IRQ_TYPE_LEVEL_HIGH>;
-				clocks = <&clks IMX6SL_CLK_ENET_REF>,
+				clocks = <&clks IMX6SL_CLK_ENET>,
 					 <&clks IMX6SL_CLK_ENET_REF>;
 				clock-names = "ipg", "ahb";
 				status = "disabled";