drivers: systick: fix cycle count in sys_clock_set_timeout()

With this patch the sys_clock_set_timeout function counts the cycles
elapsed while computing the systick timer's new load (tickless mode).
This cycles are then added to the total cycle count instead of being
lost.

This patch mitigates uptime drifting in tickless mode (especially when
high frequency timers are registered).

Signed-off-by: Simon Guinot <simon.guinot@seagate.com>