ARM: dts: imx53-voipac-dmm-668: Fix memory node duplication

imx53-voipac-dmm-668 has two memory nodes, but the correct representation
would be to use a single one with two reg entries - one for each RAM chip
select, so fix it accordingly.

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>