timer: cortex-m systick: add idle timer

Some chips, that use Cortex-M SysTick as the system timer, disable a
clock in a low power mode, that is the input for the SysTick e.g.
STM32Fx family.

It blocks enabling power management for these chips. The wake-up
function doesn't work and the time measurement is lost.

Add an additional IDLE timer that handles these functionality when the
system is about to enter IDLE. It has to wake up the chip and update the
cycle counter by time not measured by the SysTick. The IDLE timer has to
support counter API (setting alarm and reading current value).

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