powerpc/mpc85xx:Fix "boot page TLB" entry size for NAND SPL

e500v2 processor does not support 8K page size TLB entries.

So create new TLB entry only during NAND SPL boot.

Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>