[POWERPC] ipic: ack only for edge interrupts

Only external interrupts in edge detect mode support ack operation.
Therefore, in most cases ack is not needed.  The patch makes ipic
ack only when it's needed.  This could boost over all system performance.

Signed-off-by: Li Yang <leoli@freescale.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>