powerpc/time: Add set_state_oneshot_stopped decrementer callback

If CONFIG_PPC_WATCHDOG is enabled we always cap the decrementer to
0x7fffffff:

       if (IS_ENABLED(CONFIG_PPC_WATCHDOG))
                set_dec(0x7fffffff);
        else
                set_dec(decrementer_max);

If there are no future events, we don't reprogram the decrementer
after this and we end up with 0x7fffffff even on a large decrementer
capable system.

As suggested by Nick, add a set_state_oneshot_stopped callback
so we program the decrementer with decrementer_max if there are
no future events.
Signed-off-by: Anton Blanchard <anton@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/time.c

@@ -111,6 +111,7 @@ struct clock_event_device decrementer_clockevent = {
 	.rating			= 200,
 	.irq			= 0,
 	.set_next_event		= decrementer_set_next_event,
+	.set_state_oneshot_stopped = decrementer_shutdown,
 	.set_state_shutdown	= decrementer_shutdown,
 	.tick_resume		= decrementer_shutdown,
 	.features		= CLOCK_EVT_FEAT_ONESHOT |