[POWERPC] kexec: MPIC ack interrupts at mpic_teardown_this_cpu()

We really need to ack interrupts at mpic_teardown, since
not all platforms reset mpic at kernel start-up. For example,
kexec'ed kernel hangs on P.A. Semi if mpic_eoi() isn't called.

Signed-off-by: Valentine Barshak <vbarshak@ru.mvista.com>
Signed-off-by: Olof Johansson <olof@lixom.net>