include: dt-bindings: stm32_clocks: add stm32l4+ bindings header

On stm32l4+ series, the SAI1 and SAI2 clock source selection fields
are within a different register hence a different binding header
file is introduced to allow proper field usage on those platforms.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>