LF-2769-1 mx7dsabresd: Reserve top 1MB DDR for RPMSG usage

LF-2769-1 mx7dsabresd: Reserve top 1MB DDR for RPMSG usage

When booting with M4 RPMSG demo in u-boot, the M4 will use top 1MB
DDR for RPMSG vring buffer. This overlaps with u-boot MMU table and
modifies some MMU entries.
On mx7dsabresd, this cause u-boot failed to access LCDIF registers
due to the wrong MMU entries.

Signed-off-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
(cherry picked from commit 8a03d17c92cc04765c6b93f716ea081486fd15f0)
(cherry picked from commit b44fab8da91e4f74a3ab7e537368644ea1a25873)