ARM: dts: imx6: add mmdc ipg clock

i.MX6 SoCs has MMDC clock gates in CCM CCGR, add
clock property for MMDC driver's clock operation.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6ul.dtsi

@@ -917,6 +917,7 @@
 			mmdc: mmdc@21b0000 {
 				compatible = "fsl,imx6ul-mmdc", "fsl,imx6q-mmdc";
 				reg = <0x021b0000 0x4000>;
+				clocks = <&clks IMX6UL_CLK_MMDC_P0_IPG>;
 			};
 
 			weim: weim@21b8000 {