MLK-18939 mx7ulp_evk: Fix mtest end address issue

MLK-18939 mx7ulp_evk: Fix mtest end address issue

When TEE is enabled, the high 32MB memory is reserved to TEE. So the
mtest end address 0x9e000000 will overlay with u-boot runtime text and data.

Fix the issue by changing the mtest end to half of DDR size.

Signed-off-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit 9b56f22639703bd70bfdd1de0cbdcef2b80f0bff)
(cherry picked from commit e9c1c97f64df33eaded9f78268782a246010928f)
(cherry picked from commit 87e7df87b7818052a8c915a5eaeb468d22004383)