soc/arm/silabs_exx32: select missing `CONFIG_SOC_GECKO_SERIESx`

This commit:
* adds the `CONFIG_SOC_GECKO_SERIES0` Kconfig option for Gecko Series 0
  SoCs
* selects the proper `CONFIG_SOC_GECKO_SERIESx` option where it's currently
  missing

Fixes #62806.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>