soc: gecko: Enable SWO output during SoC initialization

Enable SWO debug output during system initialization and not as part of
GPIO driver initialization. After the modification the logger output
becomes available earlier during the boot process. Also, it's not
necessary anymore to build full GPIO driver only to enable SWO. This may
be critical when building small images.

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