Documentation: devicetree: Add binding for Synopsys DDR controller

Signed-off-by: NSoren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>

Documentation/devicetree/bindings/memory-controllers/synopsys.txt

+Binding for Synopsys IntelliDDR Multi Protocol Memory Controller
+
+Required properties:
+ - compatible: Should be 'xlnx,zynq-ddrc-a05'
+ - reg: Base address and size of the controllers memory area
+
+Example:
+	memory-controller@f8006000 {
+		compatible = "xlnx,zynq-ddrc-a05";
+		reg = <0xf8006000 0x1000>;
+	};