drivers: timer: stm32: Use dt to configure LPTIM domain clock

Instead of relying on Kconfig, use dt inputs to configure LPTIM domain
clck (LSI/lSE).
Clock control dedicated APIs are used for configuration and get the
frequency of domain clock in use.
Constants macros used previously to store frequency and time base are
converted to static global variables.

Some code was set up specifically to keep compatibility with targets
that still use Kconfig to configure domain clock. This will be removed
after a deprecation period.

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