drivers: timer: cortex_m_systick: make idle timer a Kconfig choice

The Cortex-M SysTick supported collaboration with a so-called "IDLE timer"
to work even in low-power modes, when the SoC may disable its clock.

Rename "IDLE timer" to "low-power mode timer" to better reflect what this
timer is used for, and transform the single option into a Kconfig choice
to allow other behaviors to be implemented and chosen.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>