powerpc/85xx:Avoid hardcoded vector address for IVORs

For e500 and e500v2 architecturees processor IVPR address should be alinged on
64K boundary.

in start.S, CONFIG_SYS_MONITOR_BASE is stored blindly in IVPR assuming it to be
64K aligned. It may not be true always. If it is not aligned, IVPR + IVORs may
not point to an exception handler.

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