ARM: dts: imx50: Fix memory node duplication

imx50-evk has duplicate memory nodes:

- One coming from the board dts file: memory@

- One coming from the imx50.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/imx50-evk.dts

@@ -12,6 +12,7 @@
 	compatible = "fsl,imx50-evk", "fsl,imx50";
 
 	memory@70000000 {
+		device_type = "memory";
 		reg = <0x70000000 0x80000000>;
 	};
 };