soc: silabs_exx32: add missing ifs to Kconfig.defconfig

This file is setting Kconfig options even when it is not the chosen
SoC. I noticed this because without this patch, CONFIG_SOC_GECKO_EMU=y
when building for an unrelated board with SYS_POWER_MANAGEMENT=y.

Hide any subtrees in this file when the EXX32 family isn't selected.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>