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: NMarco Franchi <marco.franchi@nxp.com>
Signed-off-by: NFabio Estevam <festevam@gmail.com>
Signed-off-by: NMarco Franchi <marco.franchi@nxp.com>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>