soc: stm32: common: Define STM32_LPTIM_CLOCK from device tree inputs

Now that:
1 - LS Clocks sources values are identical accross series
2 - We're able to extract this value from device tree

define STM32_LPTIM_CLOCK choice symbol from device tree

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>