ARM: dts: imx53: Fix memory node duplication

Boards based on imx53 have duplicate memory nodes:

- One coming from the board dts file: memory@

- One coming from the imx53.dtsi file.

Fix the duplication by removing the memory node from the dtsi file
and by adding 'device_type = "memory";' in the board dts.

Reported-by: Rob Herring <robh@kernel.org>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx53-cx9020.dts

@@ -22,6 +22,7 @@
 	};
 
 	memory@70000000 {
+		device_type = "memory";
 		reg = <0x70000000 0x20000000>,
 		      <0xb0000000 0x20000000>;
 	};