soc: silabs: s2: make order in soc Kconfig's

Made some order in the Kconfig's for silabs series 2 socs.
Made a distinction between silabs "generic family" (e.g. xg21) and silabs
"device family" (e.g. efr32mg21).

Signed-off-by: Yishai Jaffe <yishai1999@gmail.com>