[POWERPC] MPIC sys_device & suspend/resume

This adds mpic to the system devices and implements suspend
and resume for them.  This is necessary to get interrupts for
modules back to where they were before a suspend to disk.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>