soc: silabs_exx32: Add independent Kconfig options for HAL libraries

Zephyr gecko drivers depend on libraries provided by the vendor. The
same libraries may also be used directly by the application code or
RAIL library. To facilitate the latter use case scenario this commit
adds Kconfig options to independently enable compilation of vendor
HAL library modules.

Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>