ARM: dts: imx6sl: Fix memory node duplication

Boards based on imx6sl have duplicate memory nodes:

- One coming from the board dts file: memory@

- One coming from the imx6sl.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/imx6sl-warp.dts

@@ -55,6 +55,7 @@
 	compatible = "warp,imx6sl-warp", "fsl,imx6sl";
 
 	memory@80000000 {
+		device_type = "memory";
 		reg = <0x80000000 0x20000000>;
 	};