ARM: dts: imx31: Fix memory node duplication

ARM: dts: imx31: Fix memory node duplication

Boards based on imx31 have duplicate memory nodes:

- One coming from the board dts file: memory@

- One coming from the imx31.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>
Reviewed-by: Vladimir Zapolskiy <vz@mleia.com>
Tested-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>