clocksource: dw_apb_timer_of: Fix read_sched_clock

The read_sched_clock should return the ~value because the clock is a
countdown implementation. read_sched_clock() should be the same as
 __apbt_read_clocksource().
Signed-off-by: Dinh Nguyen <dinguyen@altera.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

drivers/clocksource/dw_apb_timer_of.c

@@ -108,7 +108,7 @@ static void __init add_clocksource(struct device_node *source_timer)
 
 static u64 read_sched_clock(void)
 {
-	return __raw_readl(sched_io_base);
+	return ~__raw_readl(sched_io_base);
 }
 
 static const struct of_device_id sptimer_ids[] __initconst = {