powerpc/64s: Implement system reset idle wakeup reason

It is possible to wake from idle due to a system reset exception, in
which case the CPU takes a system reset interrupt to wake from idle,
with system reset as the wakeup reason.

The regular (not idle wakeup) system reset interrupt handler must be
invoked in this case, otherwise the system reset interrupt is lost.

Handle the system reset interrupt immediately after CPU state has been
restored.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>