drivers: timer: Clean up Cortex-M systick Kconfig

Makes the choice CORTEX_M_SYSTICK_LPM dependent upon the
CORTEX_M_SYSTICK to prevent its default from showing up in
the .config of projects that do not use the CORTEX_M_SYSTICK
timer driver.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>