arch: nios2: timing: Fix timing cycles rollover

Fix arch_timing_cycles_get() to prevent overflow on the clock rollover.

The issue is observable on tests/benchmarks/wait_queues and
tests/benchmarks/sched_queues with BENCHMARK_NUM_ITERATIONS is large
enough, e.g. default 1000.

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>