powerpc/85xx:Fix NAND code base to support debugger

Update NAND code base to ovecome e500 and e500v2's second limitation i.e. IVPR
+ IVOR15 should be valid fetchable OP code address.

As NAND SPL does not compile vector table so making sure IVOR + IVOR15 points to
any fetchable valid data

Signed-off-by: Radu Lazarescu <radu.lazarescu@freescale.com>
Signed-off-by: Marius Grigoras <marius.grigoras@freescale.com>
Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>