ARM: dts: imx6qdl: Fix memory node duplication

Boards based on imx6qdl have duplicate memory nodes:
- One coming from the board device tree file: memory@
- One coming from the imx6qdl.dtsi file.

Fix the duplication by removing the memory node from
the imx6qdl.dtsi file and by adding 'device_type = "memory";'
in the board Device Tree.

Converted using the following command:
perl -p0777i -e 's/memory\@10000000 \{\n/memory\@10000000 \{\n\t\tdevice_type = \"memory\";\n/m' `find ./arch/arm/boot/dts -name "imx6*"``

Reported-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marco Franchi <marco.franchi@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>