dts/arm: stm32: Don't disable systick

In some stm32 series systick was disabled in order to
allow alternate use of lptim timer as kernel low power ticker.
Doing this, dts based definition of CORTEX_M_SYSTICK Kconfig symbol
is disabled and CORTEX_M_SYSTICK was redefined with 'default y'
in stm32 soc files which makes things more complex to handle to
alternate with LPTIM activation.

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