[C72 Mini][DTS] Add ram dimension node

arch/arm64/boot/dts/seco/seco-imx8mm-c72.dts

@@ -23,6 +23,11 @@
 / {
 	model = "SECO i.MX8MM C72 board";
 	compatible = "fsl,imx8mm-evk", "seco,imx8mm-c72","fsl,imx8mm";
+
+	memory@40000000 {
+		device_type = "memory";
+		reg = <0x0 0x40000000 0 0x80000000>;
+	};
 };
 
 &iomuxc {