[POWERPC] Reworked interrupt numbers for OpenPIC based Freescale chips

[POWERPC] Reworked interrupt numbers for OpenPIC based Freescale chips

Make the interrupt numbers match the OpenPIC spec intead of the
Freescale docs which distinguish between internal and external interrupts.

Now we can use the interrupt number directly to find the register offset
associated with it.

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