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>