boards: kconfig: Do not assign promptless SOC_FAMILY_EXX32

Assignments have no effect on promptless symbols. Flagged by
https://github.com/zephyrproject-rtos/zephyr/pull/20742

.

This symbol is selected by the SOC_SERIES_EFM32HG,
SOC_SERIES_EFM32PG12B, SOC_SERIES_EFM32WG, SOC_SERIES_EFR32FG1P, and
SOC_SERIES_EFR32MG12P symbols.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>