ARM: dts: add new compatible string for i.MX6QP mmdc

MMDC has a slightly different programming model between imx6q and imx6qp
in terms of perf support, it's exactly same for suspend support, so we
have fsl,imx6q-mmdc here to save patching suspend driver with the new
compatible.
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6qp.dtsi

@@ -85,5 +85,12 @@ ipu2: ipu@02800000 {
 		pcie: pcie@0x01000000 {
 			compatible = "fsl,imx6qp-pcie", "snps,dw-pcie";
 		};
+
+		aips-bus@02100000 {
+			mmdc0: mmdc@021b0000 { /* MMDC0 */
+				compatible = "fsl,imx6qp-mmdc", "fsl,imx6q-mmdc";
+				reg = <0x021b0000 0x4000>;
+			};
+		};
 	};
 };