drivers/timer: stm32: Re-instantiate warning message on lptim configuration

Following transition of lptim timer configuration from Kconfig to DT,
a warning message was set to inform users about this deprecation.
Due to errors in CI this message had to be removed while fixing the related
issues.
Now these issues are fixed, set the deprecation message again.

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