drivers: timers: systick: Enforce a barrier in _timer_cycle_get_32.

The code assumes that when the systick counter hits zero,
the timer interrupt will be taken before the loop can
read the LOAD/VAL registers, but this is not architecturally
guaranteed, and so the code can see a post-reload SysTick->VAL
and a pre-reload clock_accumulated_count, which causes it to
return an incorrectly small cycle count. By adding a ISB we
overcome this issue.

Signed-off-by: Adithya Baglody <adithya.nagaraj.baglody@intel.com>