timer: cortex_m_systick: add idle timer dependency

Allow enabling the Cortex-m idle timer only if power management is set.
It doesn't make sense to use an idle timer without PM.

It allows adding the idle timer chosen node to dts without enabling the
idle timer by default. Now, the PM config has to be set as well.

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