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

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>;

	};