Timer Systick : Handle Systick timer rollback in tickless kernel Case

In Tickeless kernel Platform timekeeping is having error because
"_sys_clock_tick_count" is not getting updated correctly.
Currently "OVERFLOW" Flag (bit 16 in timer control register)
is reset before it is taken into account into _sys_clock_tick_count.

This patch sets a flag as soon as Timer Overflow occues and clears
it when time is accounted into _sys_clock_tick_count.

Jira : ZEP-2217

Signed-off-by: Youvedeep Singh <youvedeep.singh@intel.com>