drivers: timers: systick: Enforce a barrier in _timer_cycle_get_32.
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 <06b44f6fa9a97a2b955c2d2cd070279967447860@intel.com>

zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,Vudentz/zephyr,kraj/zephyr,Vudentz/zephyr,ldts/zephyr,galak/zephyr,kraj/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,kraj/zephyr,punitvara/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,nashif/zephyr,nashif/zephyr,ldts/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,kraj/zephyr,explora26/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,ldts/zephyr,explora26/zephyr,galak/zephyr