[POWERPC] Add IRQ remapping hook

This patch adds irq remapping hook. On interrupt mechanism on Beat,
when an irq outlet which has an id which is formerly used is created,
remapping the irq is required.

Signed-off-by: Kou Ishizaki <kou.ishizaki@toshiba.co.jp>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>