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>