boards: stm32: Define lptim domain clock using device tree

Convert lptim configuration from Kconfig to device tree.
Note that some boards were using Kconfig default value (LSI),
which now has to be made explicit using device tree.

If not done already, enable the required clock node (lse/lsi).

Clean up related Kconfig bits when required.

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