drivers: timer: cortex_m_systick: add hook-based LPM timer support

Add support for a hook-based low-power mode timer to the Cortex-M SysTick
driver, in addition to the existing Counter API-based timer. This is useful
on platforms such as the STM32WB0 where the wake-up timer is too limited
for a Counter API driver to be implemented on top of it.

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