soc: silabs: siwg917: Relocate country code mapping function

Moved the `siwx91x_map_country_code_to_region()` function
from the Wi-Fi driver source file to nwp.c.

This change prepares the codebase for upcoming enhancements
related to the regulatory domain GET operation.

Signed-off-by: Arunmani Alagarsamy <arunmani.a@silabs.com>