powerpc: Mark system reset as an NMI with nmi_enter/exit()

System reset is a non-maskable interrupt from Linux's point of view
(occurs under local_irq_disable()), so it should use nmi_enter/exit.

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