clocksource: arm_generic: use this_cpu_ptr per-cpu helper

Utilize this_cpu_ptr() instead of per_cpu_ptr(...,smp_processor_id())

Signed-off-by: Shan Wei <davidshan@tencent.com>
Reviewed-by: Christoph Lameter <cl@linux.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>