soc: silabs_exx32: Remove const keyword for clock configuration

Some parameters (e.g., tuning capacitors) can be configured in the
CMU_HFXOInit_TypeDef and CMU_LFXOInit_TypeDef structures before calling
CMU_HFXOInit() and CMU_LFXOInit() during clock initialisation.

Signed-off-by: Steven Lemaire <steven.lemaire@zii.aero>