ARM: socfpga: dts: add sdram controller dt binding doc

Add binding doc for Altera SOCFPGA SDRAM controller.
Signed-off-by: Alan Tull <atull@opensource.altera.com>
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>

Documentation/devicetree/bindings/arm/altera/socfpga-sdram-controller.txt

+Altera SOCFPGA SDRAM Controller
+
+Required properties:
+- compatible : Should contain "altr,sdr-ctl" and "syscon".
+  syscon is required by the Altera SOCFPGA SDRAM EDAC.
+- reg : Should contain 1 register range (address and length)
+
+Example:
+	sdr: sdr@ffc25000 {
+		compatible = "altr,sdr-ctl", "syscon";
+		reg = <0xffc25000 0x1000>;
+	};