ppc: fix MSR_ME handling for system reset interrupt

ppc: fix MSR_ME handling for system reset interrupt

Power ISA specifies ME bit handling for system reset interrupt:

    if the interrupt occurred while the thread was in power-saving
    mode, set to 1; otherwise not altered

Power ISA 3.0, section 6.5 "Interrupt Definitions", Figure 64.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>