drivers: timer: stm32_lptim: Change clock source config check sanction

A specific check is implemented lptim driver in order to ensure global
platform clock/tick configuration is in line with recommendations.
To respect portability principles, don't error out when a config
conflict is detected but generates a warning instead.
Also, since these are only recommendations, provide an option to override
the check. Besides automatically override when ZTEST is enabled, as some
kernel tests specifically configure tick freq to 100.

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