timer: cortex_m_systick: idle timer: handle no sleep case

SysTick usually has higher measurement resolution than the IDLE timer.
When the time in low power mode is very short or 0, it is possible that
SysTick usually has measures more time since the sys_clock_set_timeout
than the idle timer.

Handle that case to keep uptime correct.

Signed-off-by: Dawid Niedzwiecki <dawidn@google.com>