soc: stm32: Enable cortex-m systick timer by default

Move systick activation in soc/ as a Kconfig.defconfig file and
remove activation in boards _defconfig files.
This will allow to deactivate it in a more flexible way
with upcoming LPTIMER as tick source when power management
features are enabled.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>