soc: silabs: Hide clock options for Series-2

Silabs Series-2 (all the EFR32xg2x chips) use the new clock driver
introduced in commit bda8ae8c ("drivers: clock_control: silabs: Add
clock control driver"). This driver get all the configuration from the
Device Tree.

The CMU (Clock Management Unit) options (CMU_HFXO_FREQ, CMU_HFRCO_FREQ,
CMU_NEED_LFXO, CMU_HFCLK_HFXO, etc...) are now only used for Series-0
and Series-1. It does not make sense to bother the users with them.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>