drivers/timers: stm32_lptim: Set LSI as default LPTIM clck source

Timer STM32 LPTIM currently supports 2 clocks sources: LSE & LSI.
LSE (external) is defined as default but its availability depends
on board support package and then may not be available.

This ends up in situations where users have LSE implicitly selected
while no crystal is available on board, leading to non functional
LPTIM.

To avoid this situation, makes LSI clock, which is always available
(since internal to the SoC), the default LPTIM source clock.
Then, default case will be functional. Users will then be able to
select LSE if needed.

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