arm:dts:nallino: Add empty reserved-memory node

The 'reserved-memory' node must exist in device tree, so that we can apply
the DT overlay to reserve memory for a custom logo.

arch/arm/boot/dts/seconorth/imx6ull-nallino.dts

@@ -44,6 +44,12 @@ memory@80000000 {
 		reg = <0x80000000 0x20000000>;
 	};
 
+	reserved-memory {
+		#address-cells = <1>;
+		#size-cells = <1>;
+		ranges;
+	};
+
 	aliases {
 		serial0 = &uart2;
 		serial1 = &uart3;