85xx: Add support for setting IVORs to fixed offset defaults

In future Book-E implementations IVORs will most likely go away and be
replaced with fixed offsets.  The IVPR will continue to exist to allow
for relocation of the interrupt vectors.

This code adds support to setup the IVORs as their fixed offset values
per the ISA 2.06 spec when we transition from u-boot to another OS
either via 'bootm' or a cpu release.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>