[POWERPC] MPIC: Minor optimization of ipi handler

Optimize MPIC IPIs, by passing in the IPI number as the argument to the
handler, since all we did was translate it back based on which mpic
the interrupt came though on (and that was always the primary mpic).

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>