MLK-25073 arm64: dts: imx8mn-ddr3l-evk-rpmsg: reserve space for Mcore

commit 84b86a513c34a7ec65500ca19e34d7d3d3219d48 from
https://source.codeaurora.org/external/imx/linux-imx



This is just a workaround for Linux 5.4 Q4 release
to avoid Linux use the memory.
This board only has 1GB memory, the 0xb8000000
exceeds the DRAM, and round back to 0x78000000,
since we not modify Mcore image, so we need to avoid
Linux touch 0x78000000 which might crash the system
and mark mcore ddr demo broken and only support booting mcore image
from U-Boot bootaux.

Reviewed-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>