arch/arm64/imx9: Boot, move mmu init to correct place

[joukkone]

MMU init must be after ddrinit.

Note: Please adhere to Contributing Guidelines.

Summary

Initialization order is wrong, mmu init must be after dram init.

Impact

Update this section, where applicable, on how change affects users,
build process, hardware, documentation, security, compatibility, etc.

Testing

Update this section with details on how did you verify the change,
what Host was used for build (OS, CPU, compiler, ..), what Target was
used for verification (arch, board:config, ..), etc. Providing build
and runtime logs from before and after change is highly appreciated.