[POWERPC] cell: hard disable interrupts in power_save()

With soft-disabled interrupts in power_save, we can
still get external exceptions on Cell, even if we are
in pause(0) a.k.a. sleep state.

When the CPU really wakes up through the 0x100 (system reset)
vector, while we have already started processing the 0x500
(external) exception, we get a panic in unrecoverable_exception()
because of the lost state.

This occurred in Systemsim for Cell, but as far as I can see,
it can theoretically occur on any machine that uses the
system reset exception to get out of sleep state.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>