dt-bindings: memory-controllers: freescale: add MMDC binding doc

Freescale MMDC (Multi Mode DDR Controller) driver is supported
since i.MX6Q, but not yet documented, this patch adds binding
doc for MMDC module driver.
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/memory-controllers/fsl/mmdc.txt

+Freescale Multi Mode DDR controller (MMDC)
+
+Required properties :
+- compatible : should be one of following:
+	for i.MX6Q/i.MX6DL:
+	- "fsl,imx6q-mmdc";
+	for i.MX6QP:
+	- "fsl,imx6qp-mmdc", "fsl,imx6q-mmdc";
+	for i.MX6SL:
+	- "fsl,imx6sl-mmdc", "fsl,imx6q-mmdc";
+	for i.MX6SLL:
+	- "fsl,imx6sll-mmdc", "fsl,imx6q-mmdc";
+	for i.MX6SX:
+	- "fsl,imx6sx-mmdc", "fsl,imx6q-mmdc";
+	for i.MX6UL/i.MX6ULL/i.MX6ULZ:
+	- "fsl,imx6ul-mmdc", "fsl,imx6q-mmdc";
+	for i.MX7ULP:
+	- "fsl,imx7ulp-mmdc", "fsl,imx6q-mmdc";
+- reg : address and size of MMDC DDR controller registers
+
+Optional properties :
+- clocks : the clock provided by the SoC to access the MMDC registers
+
+Example :
+	mmdc0: memory-controller@21b0000 { /* MMDC0 */
+		compatible = "fsl,imx6q-mmdc";
+		reg = <0x021b0000 0x4000>;
+		clocks = <&clks IMX6QDL_CLK_MMDC_P0_IPG>;
+	};
+
+	mmdc1: memory-controller@21b4000 { /* MMDC1 */
+		compatible = "fsl,imx6q-mmdc";
+		reg = <0x021b4000 0x4000>;
+		status = "disabled";
+	};