ARM: dts: imx28: Fix memory node duplication

Boards based on imx28 have duplicate memory nodes:

- One coming from the board dts file: memory@

- One coming from the imx28.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/imx28-apf28.dts

@@ -17,6 +17,7 @@
 	compatible = "armadeus,imx28-apf28", "fsl,imx28";
 
 	memory@40000000 {
+		device_type = "memory";
 		reg = <0x40000000 0x08000000>;
 	};