Documentation: devicetree: Add binding for Synopsys DDR controller

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>;

	};