The Freescale MPIC supports up to 2048 IRQs, but since we only build an
array of 168 interrupts in intr_machdep, clamp the max at 512. The most
any Freescale PowerPC chip actually supports is 452 on the T4240, so 512
is sufficient.

As part of this, increase the vector mask to the full openpic vector
mask, and use this limit as the terminator for the dispatch loop,
instead of a hard-coded 255.