ARM: dts: imx6dl-mamoj: Add a memory node

Add a memory node, with an empty memory size, which will be filled
by the bootloader.

This is done in preparation for removing the memory node from
imx6qdl.dtsi.

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