clocksource/drivers/dw_apb_timer_of: Add handling for potential memory leak

Add calls to disable the clock and unmap the timer base address in case
of any failures.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210322121844.2271041-1-dinguyen@kernel.org