ARM: DT: stm32f7: add sdram pin contol node

Also added DT binding doc for stm32 fmc(flexible memory controller).

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>
cc: Christophe KERELLO <christophe.kerello@st.com>
diff --git a/arch/arm/dts/stm32f746-disco.dts b/arch/arm/dts/stm32f746-disco.dts
index 07e0ca7..5846b0d 100644
--- a/arch/arm/dts/stm32f746-disco.dts
+++ b/arch/arm/dts/stm32f746-disco.dts
@@ -1,5 +1,6 @@
 /*
  * Copyright 2016 - Michael Kurz <michi.kurz@gmail.com>
+ * Copyright 2016 - Vikas MANOCHA <vikas.manocha@st.com>
  *
  * Based on:
  * stm32f469-disco.dts from Linux
@@ -76,6 +77,12 @@
 	status = "okay";
 };
 
+&fmc {
+	pinctrl-0 = <&fmc_pins>;
+	pinctrl-names = "default";
+	status = "okay";
+};
+
 &mac {
 	status = "okay";
 	pinctrl-0 = <&ethernet_mii>;