driver: timer: stm32_lptim: only set LSE drive if supported

Not all platforms support setting the LSE driving capability, causing
the build to fail for platform such as the STM32L072 if compiled with
CONFIG_STM32_LPTIM_CLOCK_LSE=y.

Adding an #ifdef guard around the call to skip it if not defined in the
HAL.

Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>