[PATCH] powerpc: Update MPIC workarounds

Cleanup the MPIC IO-APIC workarounds, make them a bit more generic,
smaller and faster.

Signed-off-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>