include: dt-bindings: stm32_clocks: Rework definitions

Rework clock definitions in order to be able to make global assumptions
on some specific clocks values such as STM32_SRC_LSE and STM32_SRC_LSI.
As such, introduce stm32_common_clocks.h

Complete that change by reworking values definition by doing a manual
enum (as enum is not possible in dt-bindings header files).

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