ARM: dts: imx6ul: Fix memory node duplication

Boards based on imx6ul have duplicate memory nodes:

- One coming from the board dts file: memory@

- One coming from the imx6ul.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/imx6ul-pico-hobbit.dts

@@ -53,6 +53,7 @@
 
 	/* Will be filled by the bootloader */
 	memory@80000000 {
+		device_type = "memory";
 		reg = <0x80000000 0>;
 	};