soc: silabs: Remove SOC_VENDOR_SILABS

Until now, the support for Silicon Labs SoCs is limited to the Gecko
families.

This commit allows upstreaming support for non-Gecko based SoCs produced
by Silicon Labs (i.e. SiM3U1xx).

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>