powerpc/mpc85xx: move debug tlb entry after TLB is in known state

Previously, in many if not all configs we were creating overlapping TLB entries
which is illegal.  This caused a crash during boot when moving p2020rdb NAND SPL
into L2 SRAM.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Cc: Prabhakar Kushwaha <prabhakar@freescale.com>
Cc: Andy Fleming <afleming@freescale.com>
--
Prabhakar, please test that debug still works.