soc: silabs: split in families

EXX32 does not really makes sense. Silabs tends to talk about "series
0", "series 1" and "series 2".

Note all Silabs chipsets (whatever their family) tend to share
components. So this PR introduces SOC_VENDOR_SILABS to match these
common parts.

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