stm32f7: use clock driver to enable sdram controller clock

This patch also removes the sdram/fmc clock enable from board specific
code.

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>
cc: Christophe KERELLO <christophe.kerello@st.com>
diff --git a/arch/arm/dts/stm32f746.dtsi b/arch/arm/dts/stm32f746.dtsi
index 3707550..e9fd6f4 100644
--- a/arch/arm/dts/stm32f746.dtsi
+++ b/arch/arm/dts/stm32f746.dtsi
@@ -74,6 +74,7 @@
 		fmc: fmc@A0000000 {
 			compatible = "st,stm32-fmc";
 			reg = <0xA0000000 0x1000>;
+			clocks = <&rcc 0 64>;
 			u-boot,dm-pre-reloc;
 		};