Is it for ECC or other thing?
Can those area be accessed?
Could you specify used device more concretely?
As is shown there are mirror flash memory in the Manual.
This is from the Reference Manual of MPC5744P
It is due to calibration i.e. remapping flash portion by SRAM. On the predecessors such operation has been done by MMU (using of virtual addresses). As MMU is no longer present, overlay support has been added.