drivers: timer: stm32: Specific handling for L0 LSI

On L0 series, LSI runs at 37KHz while LPTIM driver only supports speeds
up to 32768Hz (to avoid counter overflow). Consequence is a time running
faster than reality (x1.13)
Solution to this is the implementation of the LPTIM prescaler support.

While moving driver configuration from Kconfig to DT, this case was
not taken into account and the effect was LPTIM counter overflow which
consequence is worse than the slightly faster timer.
Reproduce the initial behavior with this piece of code that will be
removed once prescaler support is available.

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