powerpc/book3e-64: Add helper function to setup IVORs

Not all 64-bit Book-3E parts will have fixed IVORs so add a function that
cpusetup code can call to setup the base IVORs (0..15) to match the fixed
offsets.  We need to 'or' part of interrupt_base_book3e into the IVORs
since on parts that have them the IVPR doesn't extend as far down.

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