powerpc/xics: EOI xics ipi by hand in kexec

EOI normally has the side effect of returning the cpu to the base
priority to recieve the next interrupt.  This is actually controlled
by the top byte of the xirr register.   When we are exiting the
kernel in kexec we must eoi the ipi for the next kernel because we
never return from the handler, but we want to leave interrupt
delivery blocked until the next kernel takes action.

Since the hardware ipi vector is fixed, its easiest to just do the
eoi explicitly.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>